{4}

CHAPTER I. CELLULOSE: THE CHEMICAL PROPERTIES OF TYPICAL MEMBERS OF THE CELLULOSE GROUP, WITH REFERENCE TO THEIR NATURAL HISTORY.

Plants are so far built up of cellulose that it may be called the material basis of the vegetable world. Plant tissues, however, seldom, if ever, consist of pure cellulose, but contain besides, other products of growth either chemically combined with the cellulose or mechanically bound up with the tissue, which are, according to the nature of their union, removable either by means of fundamental chemical resolution or by the application of simple solvents. A general method for the isolation of cellulose consists in exposing the moist tissue to the action of chlorine gas or of bromine water in the cold, and subsequently boiling in dilute ammonia; repeating this treatment until the alkaline solution no longer dissolves anything from the tissue or fibre. The cellulose is then washed with water, alcohol, and ether, and dried. Obtained in this way, or in the form of bleached cotton, or of Swedish filter paper, it is a white substance, more or less opaque, retaining the microscopic features of the tissue or fibre from which it has been isolated. Its sp. gr. is 1·25–1·45. Its elementary composition is expressed by the percentage numbers (Schulze)

C 44·0 44·2
H  6·3  6·4
O 49·7 49·4

or by the corresponding empirical formula, viz. C6H10O5.

These numbers represent the composition of the ash-free cellulose. Nearly all celluloses contain a certain proportion, {5} however small, of mineral constituents, and the union of these with the organic portion of the fibre or tissue is of such a nature that the ash left on ignition preserves the form of the original. It is only in the growing point of certain young shoots that the cellulose tissue is free from mineral constituents. (Hofmeister.)

As already indicated, cellulose is insoluble in all simple solvents; it is, however, dissolved by certain reagents, but only by virtue of a preceding chemical modification. An exception to this is to be found, perhaps, in the ammoniacal solution of cupric oxide (Schweitzer’s reagent), in which it dissolves without essential modification, being recovered by precipitation, in a form which is chemically identical with the original, though differing, of course, in being structureless, or amorphous. This reagent may be employed in a variety of forms, but the following method of using it is to be recommended as the most certain in its results. The substance to be operated upon is intimately mixed with copper turnings in a tube which is narrowed below and provided with a stopcock. Strong ammonia is poured upon the contents of the tube and, after being allowed to stand for some minutes, is drawn off and returned to the tube; the operation is several times repeated until the solution of the substance is effected. In order to facilitate the oxidation of the copper by the atmospheric oxygen, a current of air may be aspirated through the apparatus. The solution of the oxide prepared in this way is more effective in its action on cellulose than that obtained by dissolving the precipitated hydrate in ammonia. Cellulosic tissues in contact with this reagent are seen to undergo a disaggregation of their fibres, which swell up, become gelatinous, and disappear in solution. On adding an acid to the viscous solution, a precipitate of the amorphous cellulose is obtained in the form of a jelly resembling hydrated alumina; after washing and drying, it forms a brownish, brittle, horny mass. The cellulose is also precipitated upon simply diluting the viscous solution with water and allowing it to stand {6} 8–10 days in a closed vessel. From this observation it was inferred by Erdmann that the cellulose could not be considered as dissolved in the strict sense of the word, but the experiments of Cramer upon the osmotic properties of the solution proved this inference to be unfounded, and that cellulose is actually dissolved by the ammoniacal solution of copper oxide.

On treating the ammonio-cupric solution of cellulose with metallic zinc, this metal precipitates the copper, replacing it in the solution, and producing the corresponding ammonio-zincic solution of cellulose, which is colourless. Some of these solutions are lævo-gyrate.

Cellulose, in those forms to which the application of the term has been hitherto restricted, is a comparatively inert substance, and its reactions are consequently few. One of these is available for the identification of cellulose, and is chiefly used in the microscopical examinations of tissues: this is its reaction with iodine. Cellulose is not coloured blue by a solution of iodine excepting under the simultaneous influence of hydriodic acid, potassium iodide, sulphuric acid, phosphoric acid, or zinc iodide or chloride. The solution is prepared in the following way: zinc is dissolved to saturation in hydrochloric acid, and the solution is evaporated to sp. gr. 2·0; to 90 parts of this solution are added 6 parts potassium iodide dissolved in 10 parts of water; and in this solution iodine is dissolved to saturation. By this solution cellulose is coloured instantly a deep-blue or violet. For the identification of cellulose in the gross, mere inspection is usually sufficient; confirmatory evidence is afforded by an observation of the action of the ammonio-copper reagent, and of the absence of reaction with chlorine water. (See p. 18.)

Cellulose in its earlier stages of elaboration has no action upon light; but with age it acquires the property of double refraction, not, as has been shown by experiment, by virtue of its state of aggregation, but of its molecular constitution (Sachs). {7}

Animal Cellulose.

—The mantles of many of the mollusca, e.g. the Pyrosomidæ, Salpidæ, and Phallusia mammillaris, contain a resistant substance which, after isolation by chemical treatment, has been found to be identical both in composition and properties with vegetable cellulose. Cellulose has also been stated to occur in degenerated human spleen and in certain parts of the brain.

Compounds of Cellulose.

—The chemical inertness of cellulose is a matter of everyday experience in the laboratory, where it fulfils the important function of a filtering medium in the greater number of separations of solids from liquids. Its combinations with acids and with basic oxides are, as might be expected, few and of little stability. It has been shown by Mills that cellulose (cotton) in common with certain other organic fibrous substances, when immersed in dilute solutions of the acids or basic oxides, condenses these bodies within itself at the expense of the surrounding solution, which is proportionately weakened. This effect of concentration is sufficiently uniform and constant to lead us to assign it to a chemical cause, and the view is strengthened by a consideration of the relative effects upon the various acids and bases which have been investigated, and brought to the following numerical expression:—Weight of cotton 3 grm. (with 6·9 per cent. H2O and 0·05 per cent. ash)—i.e. 2·893 anhydrous fibre. Strength of solution about 0·5 grm. of the reagent in 250 cc.
Temp. Time. Weight absorbed.
H2SO4 4° C. (39° F.) 3 min 0·00495
HCl „ „ 0·00733
NaOH „ „ 0·02020

The molecular ratio of the absorption, in the two latter, is 3 HCl : 10 NaOH, and it is noteworthy that the same ratio was observed for silk.

Cellulose removes barium hydrate from its solution in wafer to form with it an insoluble compound. On adding lead acetate to the solution of cellulose in the ammonio-copper reagent, so prepared as to contain no carbonate, a {8} precipitate is obtained consisting of a compound of cellulose with lead oxide, but in variable proportions. The compound C6H10O5PbO is formed by the action of finely-divided lead oxide upon the above solution. Quite recently it has been shown (O’Shea, Chem. News, May 28th, 1886) that when dilute solutions of lead are passed through ordinary filter paper, a certain amount is retained which cannot be removed by washing.

Cellulose does not combine with metallic salts, a fact which has been established incidentally to researches upon the mode of action of mordants.

The combinations of cellulose with acid radicles (ethereal salts) are both definite and stable.

Triacetyl Cellulose [C6H7 (C2H3O)3 O5] is formed by treating cellulose with six times its weight of acetic anhydride at 180° C. (356° F.). The product of the reaction is a syrupy solution from which the compound in question separates on dilution with water as a white flocculent precipitate.

Triacetyl cellulose is insoluble in alcohol and in ether, but soluble in glacial acetic acid. It is easily saponified by boiling with a solution of potassium hydrate, the cellulose being regenerated. No derivative containing more than three acetyl groups has been obtained; but a mixture of the mono-and di-acetyl cellulose is formed in treating cellulose with only twice its weight of acetic anhydride, the formation of these bodies being unattended by their solution.

Whenever cellulose, in any form, is brought into contact with strong nitric acid at a low temperature, a nitro product, or a nitrate, is formed. The extent of the nitration depends upon the concentration of the acid, on the time of contact of the cellulose with it, and on the state of the physical division of the cellulose itself.

Knop, and also Kamarsch, and Heeren, found that a mixture of sulphuric acid and nitric acid also formed nitrates of cellulose; and still later (1847), Millon and Gaudin employed a mixture of sulphuric acid and nitrates of soda or potash, which they found to have the same effect. {9}

Several well characterised nitrates have been formed, but it is a very difficult matter to prepare any one in a state of purity, and without admixture of a higher or lower nitrated body.

The following are known:—

Cellulose and Chlorine.

—Dry chlorine gas has no action upon cellulose, but the presence of water determines an indirect oxidising action. This oxidising action results from the de­comp­o­si­tion of water by chlorine with formation of hydrochloric acid and liberation of oxygen. The reaction, however, can only take place in the presence of light.

Cellulose and Oxygen.

—When cotton is exposed for some time to the action of a solution of bleaching powder, in the cold, and with access of air, e.g. when a piece of white calico is moistened with the solution, squeezed out and exposed to the air, it is found to undergo gradual disintegration accompanied by a change in composition, and an elementary {11} analysis of the product shows it to contain less carbon and more oxygen than the original cellulose. The following percentage numbers indicate the composition of these oxidised derivatives of cellulose, and the progress of the oxidation:—
12 hours’ exposure. 24 hours’ exposure.
C 43·78  43·47 43·00  42·90
H  5·85   6·13  6·28   6·18
O 50·37  50·40 50·72  50·92

Other oxidising agents produce similar results; even by exposure to air and light, cellulose is slowly converted into these oxidised derivatives.2 From their mode of formation, they have been termed oxy­cel­lu­loses, and to distinguish them from a series of more highly oxidised derivatives, produced by the action of nitric acid upon cellulose, which they nevertheless resemble in many of their characteristics, the prefix α is employed. The following are the distinguishing features of the α oxycelluloses as represented by the more extreme of the above mentioned products. It reduces Fehling’s solution at the boiling temperature, and the cuprous oxide is deposited upon the fibre in a state of intimate union, producing the effect of an orange dye. It attracts the basic colouring matters from their solutions and is dyed to a full shade, the depth of colour being proportionate to the amount of oxidation to which the cellulose has been subjected. See also p. 43. Treated with a warm solution of phenylhydrazine salts in water, it is coloured a bright lemon-yellow. Its most remarkable property is its attraction for the vanadium compounds, which is so powerful that combination may be proved to take place when this element in the form of chloride is presented to the oxycellulose in an aqueous solution containing not more than 1 in 1,000,000,000,000 parts.

2 Witz. Bull. Soc. Ind. Rouen, X. 416, and XI. 189.

The β oxycellulose resulting, as already indicated, from the action of dilute nitric acid upon cellulose, will be subsequently considered, under the head of the de­comp­o­si­tions of cellulose, to which the reaction which we have been considering may be regarded as transitional. {12}

Decomposition of Cellulose.

—Acids.—Of the de­comp­o­si­tion of cel­lu­lose by artificial means, i.e. by the action of reagents, the most important are those which disclose the relationship which it bears to the group of so-called carbo-hydrates. Cellulose is, indeed, a highly elaborated carbo-hydrate, differing from dextrin and starch, not so much in its essential constitution as in the molecular arrangement which has been impressed upon it in and through the life of the plant. There are numerous observations in biological science which demonstrate the close connection, and in many cases the physiological equivalence of the members of this natural group, which includes the sugars, dextrin, starches, and cellulose. The following facts show the constitutional or chemical ties which underlie their interdependence.

Amyloid.

—When cotton is treated with 30 times its weight of sulphuric acid somewhat diluted (1 part H2O, 4 parts H2SO4) it quickly dissolves, the solution being at first gelatinous, becoming afterwards syrupy. On pouring this solution into water, a white flocculent precipitate is formed, consisting of a substance closely resembling the cellulose from which it is derived, differing, indeed, only in being amorphous, and in being coloured blue by iodine. This substance, from its resemblance to starch, has been called amyloid. When sufficiently dehydrated it constitutes a swollen starchy mass, which dries to a brittle transparent solid. This transformation appears to consist in a simple hydration of the cellulose molecule, thus:—
2 (C6H10O5)
Cellulose.
+
H2O
=
C12H22O11.
Amyloid.

The modification of cellulose, which occurs on the conversion of unsized paper into the so-called parchment paper, by exposure for a short time to the action of strong sulphuric acid, and subsequent washing and drying, consists doubtless in a superficial conversion of the cellulose into amyloid, or a body closely resembling it.

Hydracellulose.

—Closely related in composition to the above {13} described amyloid, is the substance called hydracellulose, into which cellulose is converted by exposure for some time to the action of moderately strong sulphuric, or hydrochloric acids (sp. gr. 1·35). This substance is also formed by the action of very weak acids, if the exposure of the cellulose be sufficiently prolonged. Its formation is accompanied by the complete disintegration of the cellulose. If the action of strong sulphuric acid be prolonged, a further modification ensues, dextrin or sugar being formed. These products have not been sufficiently studied to have established their exact identity with any of the substances known under these group-names. The dextrin has a close resemblance to the dextrin obtained from starch; the sugar is susceptible of fermentation; but beyond this nothing specific is known of these bodies. Their general properties, however, and mode of formation, sufficiently establish the fact that cellulose itself is constituted of similar molecules.

The action of zinc chloride solution upon cellulose is similar to that of sulphuric acid.

Cellulose in contact with iron, and in presence of air and moisture, is converted into a sugar and a gummy substance, which latter is converted into a sugar on boiling with dilute acids. Oxidised by potassium permanganate or bichromate in presence of acetic acid, it is converted into glucose, dextrin, and formic acid.

Cellulose and nitric acid: β Oxycellulose.*

—On digesting cellulose with nitric acid (sp. gr. 1·3) at 100° C. (212° F.), a considerable quantity of oxalic acid is formed; but after prolonged digestion, a residue remains, which yields but very slowly to the action of the acid. This substance, which is white and flocculent, when thrown upon a filter and washed with water, combines with the latter to form a gelatinous hydrate. It dissolves in dilute alkalis, but without forming a stable compound with the base. It usually amounts to about 30 per cent. of the cellulose acted upon. Its elementary composition is expressed by the formula {14} C18H26O16 (C = 43·4, H = 5·3 per cent.). Treated with a mixture of concentrated sulphuric and nitric acids, it dissolves, and, on pouring the solution into water, the compound C18H23O13(NO3)3 separates as a white flocculent precipitate. In its essential properties therefore β oxycellulose exhibits a close resemblance to cellulose itself. This oxycellulose is distinguished from the members of the cellulose group by dissolving in concentrated sulphuric acid with development of a pink colour. It appears to stand in very close relationship to the group of pectic substances, which are oxidised derivatives of cellulose.

* Cross and Bevan, Chem. Soc. Journ., xliii. p. 23.

Alkalis.

—Concentrated solutions of the caustic alkalis in the cold attack and disintegrate cellulose but slowly. The action is in many cases merely superficial. Dilute solutions of the alkalis are without action upon cellulose even at the boiling temperature, unless the condition of oxidation is super-added; in this case, acid bodies are found. The extreme product of the oxidation of cellulose in presence of the caustic alkalis is oxalic acid; this acid has indeed been prepared on the commercial scale by the “fusion” of cellulosic substances, with mixture of sodium and potassium hydrates. Malic acid has been obtained as an intermediate product of this decomposition. By potassium permanganate in presence of potassium hydrate, both in solution, finely divided cellulose is oxidised to a syrupy mixture of acids resembling in composition and properties Frémy’s meta-pectic acid. A similar decomposition is effected by concentrated solutions of the hypochlorites, employed cold.

Water at High Temperature.

—Cellulose itself is only slightly affected by exposure to a temperature of 200° C. (392° F.) but in contact with water at this temperature, in sealed glass4 most part into dark brown insoluble products, a portion, however, passing into solution in the water, imparting to it a deep yellow colour. A large quantity of carbonic acid is {15} formed, and the solution contains in addition to formic acid an appreciable quantity of pyrocatechol. The formation of this aromatic substance from cellulose is, perhaps, the most noticeable feature of the decomposition.

4 The action of alkaline substances dissolved from the glass must be taken into account in this decomposition.

Decomposition by means of ferments, &c.

—The soluble ferment of the foxglove has been found to convert cellulose into glucose and dextrin. The fluid from the vermiform appendix of the rabbit has also been found to digest cellulose with liberation of marsh gas and formation of a soluble compound which reduces cupric oxide in alkaline solution.

Humus.

—In the natural decay of cellulosic structures a large quantity of marsh gas is formed, and solid products are formed, which are usually known as humic or ulmic substances. These bodies are amorphous, are largely dissolved by alkaline solutions, and are attacked by chlorine gas in presence of moisture, yielding yellow coloured chlorinated derivatives whose composition has a general resemblance to the chlorinated compounds obtained from the compound celluloses, to be hereafter described.

The most important part which ulmic substances play in the economy of nature, is in the composition of soils. It is doubtful whether they act directly as fertilisers, but by their action upon the mineral constituents of soils they contribute to the supply of these necessary elements of growth to the plant.

Synthesis of Cellulose.

—A transformation of certain soluble carbo-hydrates into cellulose, which may be regarded as a proximate synthesis of cellulose, has recently been brought to light. This is set up spontaneously in beet-juice, and results in the formation of a hard white substance, insoluble in boiling dilute potash, soluble in the ammonio-copper reagent, converted by sulphuric acid into dextrin and sugar, and by nitric acid into oxalic acid. On adding some of these cellulose lumps to a solution of pure cane-sugar, a further transformation of the saccharose into the same substance is brought about. At the same time there is simultaneously produced a gummy substance, which is precipitated by {16} alcohol as a white amorphous caoutchouc-like mass, of the same composition as cellulose, but swelling up with water, and otherwise differing in its physical properties from cellulose. This latter substance is also formed by the action of diastaste upon a solution of saccharose.

The process of cellulosic fermentation may be represented by the equation:

n. (C12H22O11)
Saccharose.
=
n. (C6H10O5)
Cellulose.
+
n. (C6H12O6)
Glucose.

A similar transformation takes place under the influence of certain fatty seeds, e. g. those of rape and colza; and it is probable that the formation of cellulose in living plants may take place at the expense of saccharose and under the influence of ferments. In support of this it has been established that in the sugar-cane, the formation of wood—i.e. cellulose—is accompanied pari passu by a decrease in saccharose. More recently, A. Brown (Chem. Soc. Journ., 432, 1886) has investigated the formation of cellulose by the “vinegar plant” growing in solutions of the carbo-hydrates, e. g. dextrose in yeast-water. The cells elaborate an extra-cellular fibrin, which acts as a “cell-collecting medium,” and they possess therefore a two-sided activity, i.e. the property above mentioned, in addition to their strictly fermentative activity. The cellulose film in question was found to contain 50 to 60 per cent. of pure cellulose. It is noteworthy that in a solution of levulose the growth of the “plant” is unattended by fermentative action, 33 per cent. of the substance being, on the other hand, transformed into cellulose.

Modifications of Cellulose in the Plant. Compound Celluloses.

—It has been already stated that cellulose occurs in nature for the most part in a form different from that in which we have hitherto considered it, viz. either combined or mixed with other substances which are of such a nature that they may be removed by a chemical process, more or less simple. One of the most characteristic of these modified celluloses constitutes the jute fibre, the isolated bast of an Indian plant, {17} which has an enormous industrial application for the coarser textile purposes. The chemistry of jute throws light on the course of lignification, the process by which cellulose is modified into wood, and in studying it in preference to wood we have the advantage of studying a simple tissue.

Like cellulose, jute dissolves in cuprammonia, and is similarly acted upon by the concentrated acids. By nitric acids it is converted into nitric ethers, which are yellow coloured, but in other respects closely resemble the pyroxylins. They are entirely soluble in acetone.

Jute differs from cellulose in the following respects: its percentage composition (excluding ash) is

C 47·0 48·0 per cent.
H  5·9  5·7 „
O 47·1 46·3 „

It is harsher to the touch, and its colour varies from grey to brown; it combines directly with the greater number of the organic colouring matters, removing them from solution, i.e., becoming dyed with them; it is coloured deep yellow by immersion in a solution of aniline sulphate; moistened with a solution of phloroglucol and afterwards with hydrochloric acid, it gives a deep red coloration; with pyrrol also in presence of hydrochloric acid it gives a deep carmine colour; it is attacked and partially converted into soluble products by a number of reagents which have no action, under similar conditions, upon cellulose. Certain of these we must consider more in detail.

Iodine.

—Jute absorbs iodine from its solution in potassium iodide. This action is characteristic of all lignified tissues, the quantity of iodine taken up being proportional to the amount of lignification. If the physical conditions remain the same, the amounts of iodine absorbed will be found to be constant for each particular fibre. This constancy points to the formation of a definite chemical compound, probably resembling the blue compound of starch and iodine. This property may be made available for the estimation of the {18} amount of mechanical wood-pulp in papers; it will be more fully considered elsewhere.

Bromine.

—Jute, exposed to the action of bromine water in the cold, combines with the halogen, being oxidised at the same time, and the products may be dissolved by boiling the modified fibre in weak alkaline solutions. On repeating this treatment four or five times, a residue of cellulose is ultimately obtained, amounting to 72 or 73 per cent. of the original fibre: the cellulose is obtained in the form of the disintegrated fibre elements. On allowing the halogen to act in presence of an alkali, i.e., as hypobromite, not only is its action intensified and accelerated, but its modus operandi more clearly revealed. The chief products are in this case, carbonic acid, bromoform CHBr3 (which may be isolated by distillation), and cellulose. At the same time the yield of cellulose is considerably less than by the former method, the cellulose itself being attacked by the hypobromite.

Chlorine.

—Dry chlorine gas has no action upon jute even when the temperature is raised to 140° C. (284° F.), but the presence of moisture determines rapid combination, at ordinary temperature, accompanied by evolution of heat. The colour of the jute changes to a bright yellow, which, on treatment of the fibre with a solution of sodium sulphite, gives place to a pure magenta. This reaction is characteristic of all lignified tissues, and is a valuable aid in the micro-chemical examination of plant structures. For this purpose, the reagent may be employed in the form of chlorine water; the sections or small fragments to be examined are allowed to remain in this solution a short time, and then mounted for observation in a dilute solution of sodium sulphite. By combination with chlorine the fibre substance or ligno-cellulose is partially resolved. Alcohol dissolves away from the chlorinated fibre, a bright yellow substance, which is precipitated in yellow flocks on pouring the solution into water. This chlorinated derivative has the composition C19H18Cl4O9. By this formula, as well as by its colour reactions, and by its yielding protocatechuic acid on fusion with potash, it is {19} shown to be related to the trihydric phenols, and to the tannins which are derivatives of these.

After dissolving away this compound by exhaustive treatment with alcohol, the fibre still gives a brilliant reaction with sodium sulphite, showing that a portion is still held back in combination with the cellulose, or cellulose residue. On treating the residual fibre with boiling nitric acid, a considerable quantity of chloropicrin CCl3NO2 is formed, and it is probable that the union of the molecule C19H18Cl4O9 with the cellulose may be effected by the aldehyde5 molecule CCl3COH; each group being contained in the original lignocellulose, the action of the chlorine tending to disturb the atomic equilibrium and to rearrange the atoms into groups, which in their modified form have less mutual coherence. If the chlorinated fibre be directly boiled with the sodium sulphite solution, its resolution into cellulose and soluble non-cellulose derivatives is complete: and this treatment constitutes the most simple and rapid method of estimating the cellulose in lignified tissues. It is only necessary finally to wash the cellulose with hot water containing a little acetic acid (when placed in a funnel it acts as its own filter), and then with, alcohol, when it may be dried and weighed. In order to ensure the resolution of the fibre by a single chlorination (in the case of jute, and the like), it must, previously to exposure to the gas, be boiled in a dilute (1 per cent.) solution of potassic hydrate. In the case of wood and other more resistant structures, it will be necessary to repeat the chlorination.

5 Furfural may also be isolated from the chlorinated jute; the survival of this aldehyde is noteworthy.

The percentage of cellulose yielded by this method is, in the case of jute, usually 2–3 per cent. higher than by the bromine method. Moreover, if the temperature be maintained at 0° C. (32° F.), by placing the fibre, which is to be exposed to the action of the gas, in contact with pounded ice, the percentage may be still further increased, amounting {20} in some cases to 80–82. Corresponding to this increased yield, the cellulose is obtained in long filaments. It will be seen, therefore, that the cellulose isolated by chemical treatment from a compound cellulose is affected both in character and quantity by the process employed, and it is affected in a much greater degree than the cellulose itself exposed to the same treatment, after isolation. The composition of the cellulose obtained in this way differs from that of celluloses, such as cotton, which exist in the plant in an isolated and more fully formed condition; it contains 43 per cent. C and 6 per cent. H, corresponding to the formula n [3 C6H10O5H2O.] The composition of this cellulose will be seen to be identical with that of certain of the oxycelluloses previously described; and its properties are, moreover, those of an oxycellulose. These facts go to show that the jute fibre substance, and the substances allied to it, are compounds of cellulose with other molecules, i.e. they are compound celluloses. They may be conveniently grouped under the term ligno-cellulose.

Dilute Mineral Acids.

—Under the action of dilute sulphuric or hydrochloric acid (5–10 per cent.) at 60°–80° C. (140°–176° F.), a considerable portion of the fibre substance is dissolved. The action of either reagent is of course purely hydrolytic; and the hydrolysis is of the simplest character, since the substance dissolved as well as the residual fibre are found to have the same percentage composition. Moreover, the residual fibre, though having undergone a profound structural modification, and having been converted into a disintegrated mass of brittle fragments, still preserves the chemical characteristics of the original, unaltered. Thus it is converted by the action of chlorine into a similar complex of derivatives, yielding on resolution by sodium sulphite solution 75 per cent. of cellulose; also when acted upon by alkaline hypobromite it yields bromoform. Under the action of these hydrolytic reagents, therefore, the fibre shows itself to be essentially a chemical whole. At the boiling temperature, these acids resolve as well as dissolve the fibre substance, and the volatile products are acetic acid and the peculiar aldehyde {21} furfural C4H3O.COH. The yield of this aldehyde may be raised with suitable precautions to 15–20 per cent. of the raw fibre. The action of dilute nitric acid upon the fibre differs from that of the above-mentioned acids, in effecting its resolution into soluble derivatives and insoluble cellulose; the yield of the latter when the reaction is conducted at 60° C. (140° F.), with 5 parts HNO3 is 60 per cent. only, showing that in the process of isolation some of the cellulose itself is attacked and converted into soluble products.

Schulze’s method of isolating cellulose from wood, and from the ligno-celluloses generally, consisted in macerating them for 12 to 14 days with 8⁄10 their weight of potassium chlorate dissolved in 12 parts by weight of nitric acid (sp. gr. 1·10), and completing the resolution by afterwards boiling with dilute ammonia. By the action of a more concentrated acid, 50 per cent. HNO3, at 80° C. (176° F.), a soluble derivative is obtained which has the composition (C25H40NO25). It is a syrupy, highly acid body, dyeing animal fibres a deep yellow shade, and giving with the earthy bases salts of the formula (C25H32NO25M4), which are precipitated by alcohol from their aqueous solution in the form of bright yellow flocks that dry to a yellow powder.

Dilute Alkalis.

—The action of boiling dilute solutions of the caustic alkalis upon the ligno-celluloses is similar to that of the dilute acids, viz., purely hydrolytic, causing no essential change in the composition of the fibre substance.

Water at High Temperatures.

—The ligno-celluloses undergo a profound modification when heated with water at elevated temperatures. They are to a great extent converted into soluble products, of which a large proportion are the volatile bodies, acetic and formic acids and furfural. The residual fibre is found to be more or less disintegrated, and of a dark-brown colour, but preserving many of the chemical features of the original. A similar disintegration is caused by exposing the ligno-celluloses in a dry state to an atmosphere of superheated steam, the effect being much increased if the ligno-cellulose be previously bleached by any of the ordinary {22} processes, i.e., under oxidising conditions. In these decompositions oxidation, as well as hydration, plays an important part. It is found that the presence of small quantities of reducing agents, e.g., sulphites, exerts a remarkable effect in retarding and often entirely preventing these changes. It is probable that they act in obviating an initial change, which, with a cumulative reaction on the fibre, effects the profound resolutions above described. (See p. 71.)

Concentrated Hydrochloric Acid.

—When exposed to the concentrated aqueous acid at ordinary temperatures, ligno-cellulose undergoes a profound structural modification, and is largely converted into soluble bodies. The disintegrated residue reacts with chlorine similarly to the original jute, but the quantity of cellulose isolated by subsequent treatment with alkaline solutions is only 50 per cent. of the original. Since jute cellulose, after isolation, exhibits under similar treatment a much smaller conversion into soluble products, 10 per cent. as compared with 33, we have here further evidence that the cellulose is present in the fibre in a condition chemically different from that of the isolated cellulose.

Sulphuric Acid.

—On triturating the fibre with the concentrated acid, it dissolves to a brownish-purple solution; on pouring this solution into water, a substance is precipitated in dark-brown flocks; when dried it has the following composition:
C 64·4
H  4·4
O 31·2

The solution yields on distillation furfural and acetic acid.

Nitric and Sulphuric Acids.

—(1 : 2 by vol.)—By the action of these acids ligno-cellulose is converted into yellow-coloured ex­plos­ive com­pounds, preserving all the external features of the original fibre, and showing a close resemblance to the pyroxylins.

The increase in weight resulting from the fixation of the {23} nitric acid residue is approximately equal to that of cotton “nitrated” under the same conditions.

They are freely soluble in acetone. On examination they prove to be homogeneous. If the fibre be warmed with the mixture after the first reaction is completed, it dissolves. The solution is found to contain oxalic, succinic, and suberic acids, but no aromatic nitro-derivatives, whereas if ligno-cellulose contain a benzene nucleus, as has been supposed, such derivatives could not fail to be formed under these conditions.

Animal Digestion of Ligno-cellulose.

—It has long been known that the urine of herbivorous animals contains hippuric acid, or benzoyl glycocine, as a normal constituent, and it has been shown that the benzoyl radicle necessary to form this body by reacting with the glycocine of the liver, is a product of the digestion of ligno-cellulose. On the other hand, it has been shown that ligno-cellulose contains no benzene compounds as such. We have therefore, in this fact additional evidence that a portion of the complex ligno-cellulose molecule is in what we may term a potentially aromatic condition.

Distribution of Ligno-celluloses.

—As already indicated, the distribution of the ligno-celluloses is extremely wide; they are met with in every variety of vegetable tissue, and their presence is readily ascertained by the reaction of chlorination already described. Of these ligno-celluloses most closely allied to jute, we may mention the stony concretions of pears (glycodrupose) and the main tissue of fir wood (glycolignose).

Decay of Ligno-cellulose.

—When a mass of ligno-cellulose (jute) is allowed to lie for a long period in a damp state, it undergoes resolution accompanied by structural disintegration. The soluble products of this change are acid bodies of the pectic class, and astringent substances closely resembling the tannins. In this decomposition also we have evidence of the plastic condition of the ligno-cellulose molecule, of the variety of its modes of resolution, according to the various directions in which it is attacked.

Decomposition by Heat.

—The celluloses burn in the air with {24} a quiet luminous flame. When heated out of contact with the air, they are completely resolved into gaseous and volatile products on the one hand, and a residual black mass, which, although containing a high percentage of carbon, and approximating in its properties to the element itself, nevertheless contains hydrogen and oxygen in essential chemical union with the carbon, in consequence of which it is expedient to apply the term pseudo-carbon to this as to all similar substances. The volatile products of the destructive distillation of pure cellulose have not been studied, but those of certain of the ligno-celluloses (woods) have been exhaustively investigated, and have moreover, a very considerable commercial importance. They divide themselves into two groups (a) soluble in water, and containing, besides water, acetic acid, methyl alcohol, methylic acetate and acetone; (b) insoluble in water, or wood tar, containing tolyene, xylene, cymene, naphthalene, chrysene, anthracene, retene, pittacal, cedriret solid paraffins, &c. The quantity of the pseudo-carbon (charcoal) obtained is, in the case of the hard woods, about 25 per cent. of their weight. The liquid portion of the products amount to some 50 per cent., and contains about one-fifth of the carbon of the original wood; the remainder of the carbon is eliminated in the gaseous form (CO, CO2, &c.). The proportion of acetic acid is 3–4 per cent., the tar amounting to 7–8 per cent.

The coals themselves may be regarded as pseudo-carbon derivatives of celluloses, formed by a process of molecular condensation, the true nature of which remains a matter of speculation. In this view, the whole of our vast series of aromatic or benzene compounds, derived as they are from the products of the destructive distillation of coals, may be traced back to a cellulose origin.

Pseudo-carbons are obtained as products of the action of various reagents upon the celluloses, and other of the so-called carbo-hydrates. These reagents, such as sulphuric acid, act in virtue of their dehydrating power; and the recognition of this fact, together with the supposed “carbonaceous” {25} character of the product, led to the erroneous conclusion that the carbohydrates are in such decompositions simply resolved into carbon and water; a conclusion which seems to derive additional warrant from the peculiar numerical relationship which exists between the C, H, O atoms of all the members of the group. Their relation is expressed in the general formula Cn H2(n−m) O(n−m), and in the somewhat misleading term carbohydrate, which is applied to the whole group. We now know that the removal of water from these bodies by the action of dehydrating agents—including heat—follows the ascertained laws of chemical dehydration, involving molecular con­den­sa­tions and rear­range­ment, and that the pseudo-carbons are the extreme terms of a series of such con­den­sa­tions or cum­u­la­tive res­o­lu­tions. The matter, however, is not as yet sufficiently investigated to enable us to state with any preciseness the mechanism of these changes. Still this general statement will enable us to avoid many of the erroneous views which have existed on the subject, and in a measure to anticipate the results of future investigation.6

6 It is worthy of mention that by the action of chlorine in presence of water, and by the action of concentrated nitric acid upon the cannel coals, substitution derivatives are formed resembling those obtained by the action of these reagents respectively upon the ligno-celluloses.

General conclusions.

—The aldehydic character of the group of ligno-celluloses is typical of the constituents of plant tissues generally, and is, there can be no doubt, extended to the celluloses. Indeed the evidence of biological observation goes to show that the ligno-celluloses are formed, by a process of chemical modification, from some more purely cellulosic tissue, which preceded it, and with this modification there is, nevertheless, a retention of the general features of the original cellulose. In the domain of animal chemistry, recent investigation has also shown that the development of proteid tissues finds its chemical expression in their aldehydic characteristics, or as it has been put, the immediate physical cause of life is to be found in the chemical tension of aldehydes. The meaning of this expression will be readily grasped on {26} reference to the properties of the aldehydes as a group; their power of combining with oxygen; of uniting by their own molecules, to polymerise; of uniting by way of dehydration; of combining with the aromatic alcohols; a consensus of properties which shows them to possess an activity surpassing that of any other group of compounds. The evidence of biology, added to our knowledge of the properties of these substances, would lead us therefore to define cellulose as a condensed aldehyde allied to the sugars, and likewise containing alcoholic OH groups; and ligno-cellulose a compound cellulose, containing a cellulose nucleus or residue so combined with certain groups which may be included in the term non-cellulose, that it yields, under the action of reagents, products belonging either to the aromatic, the furfural, or the fatty group.

Adipo-cellulose.

—Cork or Cuticular Tissue.—From the mode of formation of cork and cuticular tissues, it has been concluded that they are also modified celluloses. The aggregate ultimate composition of cork is represented by the following percentage numbers:
C 65·7
H  8·3
O 24·5
N  1·5

Cork, however, unlike the jute fibre, is by no means chemically simple, but can be resolved by the action of mere solvents into a number of proximate constituents, such as acids, a variety of fatty bodies, nitrogeneous bodies, &c.

Cuticular tissues, such as constitute the covering of fruits, are more simple in composition; the cuticle of the apple after purification, has the following percentage composition:

C 73·66
H 11·37
O 14·97

The cuticular substance of cotton, straw, esparto, &c., are doubtless similar bodies. These numbers point to a {27} remarkable similarity in composition to the fats, and, indeed, from the results of his study of this tissue, Frémy concluded that it was a fat in everything but its physical properties, of which we may mention insolubility in alcohol and in ether, and infusibility. This tissue, however, as also cork, contains a cellulose residue, which may be isolated by any of the methods of treatment given for ligno-cellulose. Under the action of boiling nitric acid these tissues are resolved into cellulose on the one hand, and a series of fatty acids or products of their decomposition, suberic and adipic acids, &c., on the other; the latter amounting in the case of cork to 40 per cent. of the weight of the substance treated. The cellulose obtained by this treatment is but 2–3 per cent.; this number, however, represents only the amount which has survived a treatment which we know to be destructive to cellulose. If, on the other hand, cork be resolved by treatment with sodium sulphite solution at 100lb. pressure, or 166° C. (331° F.), a soft mass is obtained, preserving the structural features of the original cork, until subjected to slight pressure, when it falls to a cellular mass. From this, cellulose is isolated by any of the less drastic processes above described, and is found to amount to 9–10% of the original cork. As in ligno-cellulose, we have evidence of a transition from cellulose to the tannins, so in cork and cuticular tissue we have evidence of the metamorphosis of cellulose into fats, a fact indicated in the term adipo-cellulose, which we have applied to the compound celluloses constituting these protective plant tissues. This metamorphosis is doubtless a very complex process, and would appear to involve the formation of tannins also, at least as a subsidiary result. Still, the essential feature of the change is the production of the peculiar fat-like substances which have been described; and with due regard to the limitation pointed out, the views here advanced represent the results of the investigations of the subject as far as they have proceeded.

It may be observed that the general characteristics of the {28} adipo-celluloses as of the ligno-celluloses are those of a complex aldehyde.

Pecto-Cellulose.

—The compound cellulose described under the names adipo-cellulose and ligno-cellulose have this feature in common, that they are richer in carbon than cellulose. On the other hand there is a large number of cellulosic tissues which contain in the aggregate less carbon, i.e., in which the cellulose is combined or mixed with substances of lower carbon percentage than itself. Such tissues, or rather the substances which compose them, are resolved by simple treatment with dilute alkaline solutions into insoluble cellulose, and soluble bodies of the pectic class. Thus the purified bast of the Russian flax, i.e., the raw flax, is found to contain—
43·7 per cent. C.
5·9 „ H.

On boiling with an alkali it loses in weight about 20 per cent., the substance dissolved being identical in properties with Frémy’s pectic acid, a substance containing 42 per cent. C and 4.8 per cent. H = C16H22O15. Many other bast fibres, as well as cellulose tissues, exhibit similar properties, and although this branch of the subject has been but little investigated, there is sufficient evidence for constituting a special class of compound celluloses under this term, pecto-cellulose, having properties indicated in the type selected above.

In addition to pectic acid there exists in, or can readily be obtained from certain plants, a series of bodies of a similar nature, such as pectose, pectin, parapectic acid and metapectic acid. The most important of these are pectic acid and metapectic acid.

Pectic acid, though it rarely exists ready-formed in the plant, can readily be obtained from it by the action of weak alkalis. It is best procured by boiling the pulp of turnips with a 10 per cent. solution of sodium carbonate for about half an hour. If an acid be added to the filtered liquid, pectic acid is precipitated as a transparent colourless jelly, which {29} dries up to a transparent horny mass. By the prolonged action of alkalis or acids, pectic acid is converted into metapectic acid (C8H14O9), which may be obtained as a syrupy, strongly acid liquid.

From what we know of the properties of the members of the pectic group, and therefore of the pecto-celluloses, it will be readily seen that the latter are very liable under the action of alkalis to undergo conversion into soluble derivatives.

Conversely, a study of the action of alkalis upon plant substances, enables us to determine to what extent the latter partake of the nature of the pecto-cellulose, the importance of which to the practical paper maker we need hardly point out. For a further development of this idea (see p. 43).