Bone ash has hitherto been almost entirely consumed as a raw material for the manufacture of superphosphates; but as it is sold at from £4: 10s. to £5: 10s. per ton when containing 70 per cent of phosphates, it is, in reality, a very cheap source of these substances, and merits the attention of the farmer as an application in its ordinary state.

Of strictly mineral phosphates, a considerable variety is now in use, but they are employed exclusively in the manufacture of superphosphates, as in their natural state they are so hard and insoluble, that the plant is incapable of availing itself of them.

Coprolites.—This name was originally applied by Dr. Buckland to substances found in many geological strata, and which he believed to be the dung of fossil animals. It has since been given to phosphatic concretions found chiefly in the greensand in Suffolk and Cambridgeshire, which are certainly not the same as those described by Dr. Buckland, but consist of fragments of bones, ammonites, and other fossils. Coprolites are now collected in very large quantities, and about 43,000 tons are annually employed. They are extremely hard, and require powerful machinery to reduce them to powder, and hence their price is considerable, being about £2: 10s. per ton. Their composition varies somewhat according to the care taken in selecting them, and the locality from which they have been obtained. A general idea of their composition may be derived from the subjoined analyses:—

Within the last two or three years, coprolites have been found in great abundance in France, but they are of inferior quality, and rarely contain more than 40 per cent of phosphates.

Apatite, or mineral phosphate of lime, is found in large deposits in different places. It is particularly abundant in Spain, and occurs also in America and Norway. From the latter country it has been imported to some extent; and during the last year considerable quantities have been brought from Spain, and the importations will undoubtedly increase very largely as the means of transport improve in that country. Spanish apatite contains—

Several other varieties of mineral phosphates have been imported under the name of guano. The most important is Sombrero Island guano, which is found on a small island in the Gulf of Mexico, where it occurs in a layer said to be forty feet thick. It contains—

A somewhat similar substance, but in hard crusts, has been imported, under the names of Maracaybo guano, Pyroguanite, etc., which contains—

These substances are all excellent sources of phosphates, but they are so hard that the plants cannot extract phosphoric acid from them, and they are only useful when made soluble by chemical processes.

Superphosphate; Dissolved Bones.—These names were at first applied to bones which had been treated with sulphuric acid; but superphosphates are now rarely made from bones alone, but bone ash and some of the mineral phosphates just described are employed, either along with them, or very frequently alone. The manufacture of superphosphates depends on the existence of two different compounds of phosphoric acid and lime, one of which contains three times as much lime as the other. That which contains the larger quantity of lime is found in the bones and all other natural phosphates, and is quite insoluble in water; but when two-thirds of its lime are removed, it is converted into the other compound, which is exceedingly soluble. This change is effected by the use of sulphuric acid, which combines with two-thirds of the lime of the ordinary insoluble phosphate of lime, and converts it into the biphosphate of lime, which is soluble. When, therefore, we add to 100 lbs. of common phosphate of lime the necessary quantity of sulphuric acid, it yields 64 lbs. of biphosphate, containing the whole of the phosphoric acid, which is the valuable constituent, the diminution in weight being due to the removal of the valueless lime. Hence it follows, also, that as the lime so removed is converted into sulphate, there must, for every 100 lbs. of phosphate of lime converted into biphosphate, be produced 87 lbs. of dry sulphate of lime, or 110 of the ordinary sulphate called gypsum. This is the minimum quantity which can be present, but in actual practice it is liable to be greatly exceeded, more especially where coprolites are used, owing to the large amount of carbonate of lime they contain, which is also converted into sulphate by the action of the acid, so that it is far from uncommon to find the gypsum twice as great as it would be if materials free from carbonates could be obtained. By employing a sufficiency of sulphuric acid, the whole quantity of phosphoric acid in the bones may be thus brought into a soluble state, but in actual practice it is found preferable to leave part of it in the insoluble condition; as where it is entirely soluble, its effect is too great during the early part of the season, and deficient at its end. In order to dissolve bones, bone ash, or mineral phosphates, they are mixed with from a third to half their weight of sulphuric acid, of specific gravity 1·70 or 140° Twaddell. When mineral phosphates, and particularly coprolites, are used, the quantity of sulphuric acid must be increased so as to compensate for the loss of that which is consumed in decomposing the carbonate of lime they contain. When operating on the small scale, the materials are put into a vessel of wood, stone, or lead (iron is to be avoided, as it is rapidly corroded by the acid), and mixed with from a sixth to a fourth of their weight of water, which may with advantage be used hot. The sulphuric acid is then added, and mixed as uniformly as possible with the bones. Considerable effervescence takes place, and the mass becomes extremely hot. At the end of two or three days it is turned over with the spade, and after standing for some days longer, generally becomes pretty dry. Should it still be too moist to be sown, it must be again turned over, and mixed with some dry substance to absorb the moisture. For this purpose everything containing lime or its carbonate must be carefully avoided, as they bring back the phosphates into the insoluble state, and undo what the sulphuric acid has done. Peat, saw-dust, sand, decaying leaves, or similar substances, will answer the purpose, and they should all be made thoroughly dry before being used. An excellent plan is to sift the bones before dissolving, to apply the acid to the coarser part, and afterwards to mix in the fine dust which has passed through the sieve, to dry up the mass; or a small quantity of bone ash, of good quality, or Peruvian guano, may be used. On the large scale, mechanical arrangements are employed for mixing the materials, so as to economise labour, and mineral phosphates, such as apatite, can then be used with advantage. In such cases, blood, sulphate of ammonia, soot, and other refuse matters, are occasionally used to supply the requisite quantity of nitrogenous substances, but large quantities are also made from bone ash, etc., without these additions.

The composition of superphosphates must necessarily vary to a great extent, and depends not only on the materials, but on the proportion of acid used for solution. The following analysis illustrates the composition of good samples made from different substances—

Superphosphates made from bones alone are generally distinguished by a large quantity of ammonia, and a rather low per centage of biphosphate of lime. This is owing to the difficulty experienced in making the acid react in a satisfactory manner on bones, the phosphates being protected from its action by the large quantity of animal matter which, when moistened, swells up, fills the pores, and prevents the ready access of the acid to the interior of the fragments. Superphosphates from bone-ash, on the other hand, contain a mere trifle of ammonia, and when well made a very large quantity of biphosphate of lime. Their quality differs very greatly, and depends, of course, on that of the bone-ash employed, which can rarely be obtained of quality sufficient to yield more than 30 or 35 per cent of soluble phosphates. Coprolites are seldom used alone for the manufacture of superphosphates, but are generally mixed with bone-ash and bone dust. Mixtures containing salts of ammonia, flesh, blood, etc., are also largely manufactured, and some are now produced containing as much as four or five per cent of ammonia, and the consumption of such articles is largely increasing.

The analyses above given are all those of good superphosphates, in which abundance of acid has been used so as to convert a large proportion of insoluble into soluble phosphates; but there are many samples of very inferior quality to be met with in the market, in which the proportion of acid has been reduced, and the quantity of phosphates made soluble is consequently much lower than it ought to be. The following analyses illustrate the composition of such manures, which are all very inferior and generally worth much less than the price asked for them.

The deliberate adulteration of superphosphate, that is, the addition to it of sand or similar worthless materials, I believe to be but little practised. The most common fraud consists in selling as pure dissolved bones, articles made in part, and sometimes almost entirely, from coprolites. Occasionally refuse matters are used, but less with the intention of actually diminishing the value of the manure as for the purpose of acting as driers. It is said that sulphate of lime is sometimes employed for this purpose, but this is rarely done, because that substance is always a necessary constituent of superphosphate in very large quantities; and as farmers look upon it with great suspicion, all the efforts of the manufacturers are directed towards reducing its quantity as much as possible. It is very commonly supposed by farmers that the sulphate of lime found in so large quantity in all superphosphates, and often amounting to as much as fifty per cent, has been added to the materials in the process of manufacture, but this is a mistake; it is a necessary and inevitable product of the chemical action by which the phosphates are rendered soluble, although its quantity depends on the materials from which the manure is made. When pure bones are used its quantity is small, and it does not greatly exceed twice that of the biphosphate of lime; but in a manure made from coprolites, or other substances containing a large proportion of carbonate of lime, which must in the process of manufacture be converted into sulphate, it may be four or five times as much.

Although there is no manure which varies more in quality, or requires greater vigilance on the part of the purchaser, in order to obtain a good article, there is no doubt that superphosphates, owing to the process of manufacture being better understood, and to increased competition, have considerably improved in quality. Six or eight years since a manure containing thirty per cent of phosphates, of which twelve or fifteen had been converted into biphosphate, was considered a fair sample, but now the proportion rendered soluble is greatly increased; and where bone ash alone is employed, as much as thirty and even forty per cent of soluble phosphates is occasionally found. This, of course, is an exceptional case, and great attention and care in the selection of materials are necessary to obtain so large a proportion. The analyses already given will shew the farmer what he has to expect in good superphosphates, but it is very necessary that he should take care to obtain from the manufacturer a manure equal to the guarantee; and he ought to bear in mind that, owing to the difficulty of getting materials of constant composition, variations often take place to a considerable extent in manures which are supposed to be made in exactly the same manner.

Phospho-Peruvian Guano.—Under this name a kind of superphosphate, which is understood to be made by dissolving a native "rock guano," has recently attracted considerable attention, and is used to a large extent. Its composition is—

It is chiefly distinguished by the large proportion of valuable ingredients it contains, and the care taken to secure uniformity of composition.

A variety of substances are sold under the name of nitrophosphate, potato manure, cereal manure, etc. etc., which are all superphosphates, differing only in the proportion of their ingredients, and in the addition of small quantities of alkaline salts, sulphate of magnesia, and other substances, but they present little difference from ordinary superphosphates in their effects.

The use of superphosphate has greatly extended of late years, and its consumption has increased in a greatly more rapid ratio than that of guano or any other manure. Ten or twelve years since it was comparatively little known, but it has now come to be used in many cases in which Peruvian guano was formerly employed. It produces a better effect than that manure on light soils, although in general a mixture of the two answers better than either separately. When Peruvian guano is to be applied along with it, the farmer will naturally select a superphosphate made from bone ash, and containing the largest obtainable quantity of soluble phosphates; but when it is to be used alone, it is advisable to take one made from bones, or at all events one containing a considerable quantity of nitrogenous matter or ammonia. The kind to be selected must, however, be greatly dependent on the particular soil, and the situation in which it is to be used.

Lime.—Lime is by far the most important of the mineral manures, and an almost indispensable agent of agricultural improvement. It has been used as chalk, marl, shell and coral sand, ground limestone, and as quick and slaked lime, and its action varies according as it is applied in any of its natural forms, or after being burnt. In all of its native forms the lime is combined with carbonic acid in the proportion of fifty-six parts of lime to forty-four of carbonic acid, and the carbonate is generally mixed with variable quantities of earthy ingredients, which in some instances are important additions to it, and affect its utility as a manure.

Chalk is a very pure form of carbonate of lime, and where it abounds has been largely employed as an application on the soil. It is dug out of pits and exposed to the action of the winter's frost, by which it is thoroughly disintegrated, and in spring it is applied in quantities, which, in many instances, are only limited by the question of cost.

Marl is a name given to a mixture of finely-divided carbonate of lime, with variable proportions of clay and siliceous matters, which is found at the bottom of valleys and in hollow places in beds often of considerable extent and thickness, where it is deposited from the waters of lakes holding lime in solution, fed by streams passing over limestone, or rocks rich in lime. The composition of marls differs greatly in different districts, and they have been divided into true marls, and clay marls, according as the carbonate of lime or clay is the preponderating ingredient. The following table illustrates the composition of different varieties:—

The true marls, that is those in which carbonate of lime abounds, are greatly preferable to clay marls, the latter, indeed, operate chiefly mechanically, by altering the texture of the soil—the lime they contain being frequently too small to exercise much appreciable effect.

Shell and coral sands consist chiefly of fragments of shells and coral disintegrated by the action of the waves, and mixed with more or less siliceous sand, and containing small quantities of phosphate of lime. They occur to a considerable extent both on our own coasts and those of France, and have been used with good effect on some descriptions of soil.

The general composition of limestones has been already adverted to, when treating of the origin of soils, and a distinction drawn between the common limestones and dolomite or magnesium limestone. Few limestones can be considered as even approaching to purity, and they almost all contain a small quantity of carbonate of magnesia as well as earthy matters, and occasionally a little phosphate of lime. In good specimens the quantities of these substances are generally small, and they usually contain about half their weight of lime. When limestone is burnt in the kiln, the change which ensues consists in the expulsion of the carbonic acid, and the consequent conversion of the lime into the uncombined or quick state. If water be thrown upon it when in this condition, it becomes hot, swells up, and falls to a fine soft powder, and has then entered into combination with water. If it be exposed to the air, the same action takes place, although, of course, more slowly; and if it be left for a sufficient time, it at length absorbs carbonic acid, and reverts to its original form of carbonate of lime, although now in a state of very fine division.

While lime may be applied in the state of carbonate, either as chalk, marl, or pounded limestone, and with a certain amount of advantage, much greater effects are obtained from the use of lime itself in the quick or slaked state. These advantages are dependent partly on the mechanical effect of the burning and slaking, which enable us to reduce the lime to a much more minute state of division, and consequently to incorporate it more uniformly and thoroughly with the soil, and partly on the more powerful chemical action which it exists when in the quick or caustic state. Other minor advantages are also secured, such as the production of a certain quantity of sulphate of lime, produced by the oxidation of the sulphur of the coal used in burning, etc., which, though comparatively trifling, may, under particular circumstances and in some soils, be of considerable importance.

The action of lime is of a complicated character. Where the soil is deficient in lime, it must necessarily act by supplying that substance to the plants growing in it. But this is manifestly a very subordinate part of its action,—1st, Because no soil exists which does not contain lime in sufficient quantity to supply that element to the plants. 2d, Because its effects are not restricted to those soils in which it exists naturally in small quantity; and, 3d, Because it is found that a small application, such as would suffice for the wants of the crops, is not sufficient to produce its best effects.

It is a familiar fact that the quantity of lime applied to the soil for agricultural purposes is very large, as much as ten, and even twenty tons per acre having been used, while the smallest application is exceedingly large when compared with the mere requirements of the crops. Of late years the very large applications once in use have become less common, as it has been found preferable to employ smaller doses more frequently repeated. The quantity used depends, however, to a great extent, on the nature and condition of the soil, heavy clays, especially if undrained, and soils of a peaty nature, requiring a large application; while on well drained and light soils a smaller quantity suffices. Thin soils also require only a small application. The geological origin of the soil is also not without its influence, and its beneficial effect is peculiarly seen on granite, porphyry, and gneiss soils, both because these are naturally deficient in lime, and because the decompositions by which their valuable constituents are liberated take place with extreme slowness.

The greater part of the action of lime is unquestionably dependent on its exerting a chemical decomposition on the soil; and it acts equally on both the great divisions of its constituents, the inorganic and the organic. On the former, it operates by decomposing the silicates, which form the main part of the soil, and the alkalies they contain being thus set free, a larger supply becomes available to the plant. On the organic constituents its effects are principally expended in promoting the decomposition which converts their nitrogen into ammonia; and thus a supply of food, which might remain for a long period locked up, is set free in a state in which the plant can at once absorb it. But these chemical decompositions are attended by a corresponding change in the mechanical characters of the soil. Heavy clays are observed to become lighter and more open in their texture; and those which are too rich in organic matter have it rapidly reduced in quantity, and the excessive lightness which it occasions diminished.

The effects of an application of lime are not generally observed immediately, but become apparent in the course of one or two years, when it has had time to exert its chemical influence on the soil; but from that time its effects are seen gradually to diminish and finally to cease entirely. The period within which this occurs necessarily varies with the amount of the application and the nature of the soil, but it may be said generally that lime will last from ten to fifteen years. The cessation of its effects is due to several circumstances, partly of course to the absorption of lime by the plants, partly to its being washed out of the soil by the rains, and partly to its tendency to sink to a lower level, a tendency which most practical men have had opportunities of observing. In the latter case, deep-ploughing often produces a marked effect, and sometimes makes it possible to postpone for a year or two the reapplication of lime. All these circumstances have their influence in bringing its action to an end, but the most important is, that after a time it has exhausted its decomposing effect on the soil, having destroyed all the organic matter, or liberated all the insoluble mineral substances which the quantity added is competent to do, and so the soil passes back to its old state. It does even more, for unless active measures are taken to sustain it by other means, it is found that the fertility of the soil is apt to become less than it was before the use of lime. And that it should be so is manifest, if we consider that the lime added has liberated a quantity of inorganic matter, which, in the natural state of the soil, would have become slowly available to the plant, and that it must have acted chiefly in those very portions which, from having already undergone a partial decomposition, were ready to pass into a state fitted for absorption, and thus as it were must have anticipated the supplies of future years. This effect has been frequently observed by farmers, and is indeed so common, that it has passed into a proverbial saying, that "lime enriches the fathers and impoverishes the sons." But this is true only when the soil is stinted of other manures, for when it is well manured the exhausting effect of lime is not observed; and it must be laid down as a practical rule, that its use necessitates a liberal treatment of the soil in all other respects. But when lime has been once employed it becomes almost necessary to resort to it again; and generally so soon as its effects are exhausted a new quantity is applied, not so large as that which is used when the soil is first limed, but still considerable. When this is done very frequently, however, bad effects ensue; the soil gets into a particular state, in which it is so open that the grain crops become uncertain, and such land is said, in practical language, to be overlimed. The explanation of this state of matters commonly assumed by those unacquainted with chemistry is, that the land has become too full of lime; but a moment's consideration of the very small fraction of the soil which even the largest application of lime forms, will serve to shew that this cannot be the cause. Ten tons of lime per acre amounts to only one per cent of the soil, and as a considerable part of the lime is carried off by drainage in the course of years, it is obvious that even very large and frequently repeated doses are not likely to produce any great accumulation of that substance. In point of fact, analyses of overlimed soils have proved that the lime does not exceed the ordinary quantity found in fertile land. The explanation of the phenomenon is probably to be found in the rapid decomposition of organic matter by the lime, and its escape as carbonic acid, by which the soil is left in that curious porous condition so well known in practice. The cure for overliming is found to be the employment of such means as consolidate the soil, such as eating off with sheep, rolling, or laying down to permanent pasture.

The immediate effect of lime on the vegetation of the land to which it is applied is very striking. It immediately destroys all sorts of moss, makes a tender herbage spring up, and eradicates a number of weeds. It improves the quantity and quality of most crops, and causes them to arrive more rapidly at maturity. The extent to which it produces these effects is dependent on the form in which it is applied. When the lime is used hot, that is, immediately after it has been slaked, they are produced most rapidly and effectually; but if it has been so long exposed to the air as to absorb much of the carbonic acid it lost in burning, and has got into what is commonly called the mild state, it operates more slowly; and when it is applied as chalk, marl, or pounded limestone, its action is still more tardy. Various circumstances, which must depend upon very different considerations, must necessarily influence the farmer in the selection of one or other of these different forms of lime; but on the whole, it will be found that the greatest advantages are on the side of the well-burned and freshly slaked lime. The consideration of all the minutiæ to be attended to, however, would carry us far beyond the limits of this work, and trench to some extent on the subject of practical agriculture.

Various kinds of refuse matters containing lime have been used in agriculture, but they are generally inferior to good lime, and not generally more economical. The most important of these is gas lime, or lime which has been used for purifying coal gas. In going through this process it absorbs carbonic acid from the gas, and consequently passes back, more or less, completely into the form of carbonate of lime. But it also takes up sulphur, which remains in it in the form of sulphuret of calcium. It is well known that all sulphurets are prejudicial to vegetable life, and hence, when fresh gas lime is used, its effects are often injurious rather than beneficial. But if it be exposed for some time to the air, oxygen is absorbed, the sulphur is converted into sulphuric acid, gypsum is produced to the extent of some per cent, and the lime then becomes innocuous. When composted with dry soil, the admission of air into the interior of the lime is facilitated, and this change takes place with greater rapidity. The waste lime from bleach-works, tanneries, and other manufactories, is occasionally used by farmers; but unless obtained at a nominal price, it cannot compete with good quick lime, owing to the large amount of water it contains, and the consequent increase in the cost of carriage.

Sulphate of Lime or Gypsum.—Gypsum has been extensively used as a manure, and is found to exert a very remarkable influence upon clover, and leguminous crops generally. It is employed in quantities varying from two cwt. per acre up to a very large quantity, and almost invariably with good results, in some instances even with the production of double crops. Much speculation has taken place as to the cause of this action which is so specific in its character, and from Sir Humphrey Davy down to the present time, many chemists and agriculturists have considered the matter. Sir Humphrey Davy attributed its action to its supplying sulphur to those plants which, according to him, contain an unusually large quantity of that element. That opinion has been since entertained by others, but it can scarcely be considered as well founded, for the more accurate experiments recently made do not point to any conspicuous differences between the quantities of sulphur contained in these and other plants. It is, moreover, to gypsum alone that these effects are due, and if it were merely as a source of sulphur that it was employed, there are other salts which could be equally, perhaps more advantageously, used; such, for instance, as sulphate of soda. Others have attributed its action to its power of fixing ammonia, but this explanation is certainly untenable, for the soil itself possesses this property very powerfully, and it is inconceivable that the addition of a few hundred weights of gypsum should have any effect in promoting this action. The experiments which have been made with gypsum leave no doubt as to its effect, more especially on leguminous plants, but they do not afford an explanation of its mode of action, for which further inquiries, directed especially to that object, are required.

The application of gypsum to the soil appears to have diminished of late years, and this is probably due to the large consumption of superphosphates, and other manufactured manures, which contain it in abundance. In an ordinary application of these substances, there are contained from one to two hundredweight of gypsum; and it is not likely that when they have been extensively used, much benefit will be derived from a further application of it by itself.

CHAPTER XII.

THE VALUATION OF MANURES.

The determination of the value of a manure is in many respects a commercial rather than a chemical question, but as it must be founded on the analysis, and presents some peculiarities dependent on the complicated nature of the substances to be valued, it has fallen to some extent into the hands of the chemist. The principle on which the value of any commercial sample is estimated is very simple. It is only necessary to know the price of the pure article, and that of the particular sample to be valued is obtained by making a deduction from this price proportionate to the per centage of impurities shewn by the analysis. Thus, for example, if pure sulphate of ammonia sells at £16 per ton, a sample containing 10 per cent of impurities ought to be purchased for £14: 8s., and so on for any other quantity. This system which answers perfectly with sulphate of ammonia, nitrate of soda, or any other substance whose value depends on one individual element, is inapplicable in the case of complex manures, such as guano and the like, in which several factors combine to make up the value. In such cases, manures of very different composition may have the same value, the deficiency in one particular element being counterbalanced by the excess of another. Hence it becomes necessary to obtain an estimate of the value of each factor, from which that not only of one particular substance, but of every possible mixture may be determined.

When we come to inquire minutely into this question, it appears that the commercial value of any substance is not estimated solely by considerations of composition, but is dependent to a great extent on questions of demand and supply, and applicability to particular purposes. Thus coprolites containing from 55 to 60 per cent of phosphates sell at about £2: 12s. per ton, while bone-ash containing the same quantity of that ingredient brings about twice as much; in other words, phosphates are nearly twice as valuable in bone-ash as in coprolites, and as a phosphatic guano their price is generally still higher; and the reason for this is obvious, in bones and guano the phosphates are in a high state of division, in which they are easily attacked and disintegrated by the carbonic acid of the soil, and rendered available to plants; while in coprolites they are in a hard and compact form, and are of little use unless they have previously undergone an expensive preparation. In the same way, if the market price of different kinds of guano be inquired into, very great differences are found to exist in the rate at which phosphates are sold, and this is attributable in part to the fact that the price at which any article is charged commercially, is such as to cover the prime cost, expense of freight, and other charges, and to leave a profit to the importer; and partly, also, no doubt, to the carelessness with which manures are often purchased, and to the want of careful field experiments in which the effects produced by them are properly compared. It will be readily understood that the state of division of any substance, the readiness with which its constituents can be rendered available to the plants, care of application, and many other circumstances must influence its price; but making due allowance for these, differences are met with which appear to some extent to be merely the result of caprice. It is easy to understand why bone-ash should sell at double the price of coprolites, but no good reason can be shewn why the phosphates in one kind of guano should be sold at a much higher price than another, and the difference would probably disappear if greater attention were paid to the results of field experiments.

However great and inexplicable these differences may be, it is not the business of the valuator of a manure to discuss them. On the contrary, he is bound to accept them as the basis of his calculation, and to endeavour to deduce from them a proper system of estimation for each substance. Strictly speaking, each individual manure ought to be valued according to a plan special to itself, and deduced from its own standard market price; but it is obvious that this would lead to innumerable complications and defeat its own ends, and hence an attempt has been made to contrive a general system suited to all manures, and which, though not absolutely correct, is a sufficient approximation for all practical purposes, and a tolerably accurate guide to the determination of their relative values.

The constituents of a manure which are of actual value are ammonia, insoluble phosphates, biphosphate of lime (soluble phosphates), sulphate of lime, nitric acid (as nitrate of soda), potash, soda, and organic matter. These substances differ greatly in value. Ammonia and phosphates, soluble and insoluble, are costly; and by far the larger part of the value of all guanos, and the common manufactured manures, depends on them. Nitric acid and potash are also very valuable substances, but as they are rarely found in manufactured manures, and never in sufficient quantity to exert any material influence in their price, it is not usual to take them into consideration except in particular cases. The alkali which commonly exists in artificial manures is soda, and when alkaline salts appear in any analysis, they must be assumed to consist almost entirely of that substance generally in the form of common salt, and be valued accordingly. Sulphate of lime and organic matter though abundant constituents of most manures, add but little to their value, and it is a moot point whether they ought to be taken into consideration, although most persons allow a small value for them. Carbonate of lime, sand, or siliceous matter, and water, of course, are altogether worthless.

In order to obtain the value of a manure containing several of these substances, it is necessary to ascertain the average commercial price of each individually. This is easily done when they are met with in commerce separately, or at least mixed only with worthless substances, but some of them are only found in complex mixtures, and in these cases it is necessary to arrive at a result by an indirect process, according to methods which will be immediately explained. The question to be solved is the price actually paid for a ton of each substance in a pure state, and we shall proceed to consider them in succession.

Insoluble Phosphates.—These are purchased alone, chiefly in the form of coprolites and bone-ash, or the spent animal charcoal of the sugar refiners. Ground coprolites, containing about 58 per cent of phosphates, sell at £2: 12s. per ton, which is at the rate of £4: 8s. for pure phosphates. Bone-ash varies considerably in price, but of late samples containing 70 per cent of phosphates have sold as low as £4: 10s. per ton, and consequently pure phosphates in this form are worth £6: 8s. per ton. Although these are the only forms in which phosphates are purchased alone, it is possible to determine the price at which they are sold in bones and phosphatic guanos, by first deducting the value of the ammonia they contain, and assuming the remainder to represent the price paid for the phosphates. In this way we find the following values for insoluble phosphates:—