143. Kinds of Nitrogen in Fertilizers.—Nitrogen is the most costly of the essential plant foods. It has been shown in the first volume, paragraph 23, that the popular notion regarding the relatively great abundance of nitrogen is erroneous. It forms only 0.02 per cent of the matter forming and pertaining to the earth’s crust. The great mass of nitrogen forming the bulk of the atmosphere is inert and useless in respect of its adaptation to plant food. It is not until it becomes oxidized by combustion, electrical discharges, or the action of certain microorganisms that it assumes an agricultural value.

Having already, in the first volume, described the relation of nitrogen to the soil it remains the sole province of the present part to study it as aggregated in a form suited to plant fertilization. In this function nitrogen may claim the attention of the analyst in the following forms:

1. In organic combination in animal or vegetable substances, forming a large class of bodies, of which protein may be taken as the type. Dried blood or cottonseed-meal illustrates this form of combination.

2. In the form of ammonia or combinations thereof, especially as ammonium sulfate, or as amid nitrogen.

3. In a more highly oxidized form as nitrous or nitric acid usually united with a base of which Chile saltpeter may be taken as a type.

The analyst has often to deal with single forms of nitrogenous compounds, but in many instances may also find all the typical forms in a single sample. Among the possible cases which may arise the following are types:

a. The sample under examination may contain nitrogen in all three forms mentioned above.

b. There may be present nitrogen in the organic form mixed with nitric nitrogen.

c. Ammoniacal nitrogen may replace the nitric in the above combination.

d. The sample may contain no organic but only nitric and ammoniacal nitrogen.

e. Only nitric or ammoniacal nitrogen may be present.

144. Determination of the State of Combination.—Some of the sample is mixed with a little powdered soda-lime. If ammoniacal nitrogen be present free ammonia is evolved even in the cold and may be detected either by its odor or by testing the escaping gas with litmus or turmeric paper. A glass rod moistened with strong hydrochloric acid will produce white fumes of ammonium chlorid when brought near the escaping ammonia.

If the sample contain any notable amount of nitric acid it will be revealed by treating an aqueous solution of it with a crystal of ferrous sulfate and strong sulfuric acid. The iron salt should be placed in a test-tube with a few drops of the solution of the fertilizer and the sulfuric acid poured down the sides of the tube in such a way as not to mix with the other liquids. The tube must be kept cold. A dark brown ring will mark the disk of separation between the sulfuric acid and the aqueous solution in case nitric acid be present. If water produce a solution of the sample too highly colored to be used as above, alcohol of eighty per cent strength may be substituted. The coloration produced in this case is of a rose or purple tint.

Nitric nitrogen may also be detected by means of brucin. If a few drops of an aqueous solution of brucin be mixed with the same quantity of an aqueous extract of the sample under examination and strong sulfuric acid be added, as described above, there will be developed at the disk of contact between the acid and the mixed solutions a persistent rose tint varying to yellow.

To detect the presence of organic albuminoid nitrogen the residue insoluble in water, when heated with soda-lime, will give rise to ammonia which may be detected as described above.

145. Microscopic Examination.—If the chemical test reveal the presence of organic nitrogen the next point to be determined is the nature of the substance containing it. Often this is revealed by simple inspection, as in the case of cottonseed-meal. Frequently, however, especially in cases of fine-ground mixed goods, the microscope must be employed to determine the character of the organic matter. It is important to know whether hair, horn, hoof, and other less valuable forms of nitrogenous compounds have been substituted for dried blood, tankage, and more valuable forms. In most cases the qualitative chemical, and microscopic examination will be sufficient. There may be cases, however, where the analyst will be under the necessity of using other means of identification suggested by his skill and experience or the circumstances connected with any particular instance. In such cases the general appearance, odor, and consistence of the sample may afford valuable indications which will aid in discovering the origin of the nitrogenous materials.

SOURCES OF NITROGENOUS FERTILIZERS.

146. Seeds and Seed Residues.—The proteid matters in seeds and seed residues, after the extraction of the oil, are highly prized as sources of nitrogenous fertilizers either for direct application or for mixing. Typical of this class of substances is cottonseed-meal, the residue left after the extraction of the oil which is accomplished at the present time mostly by hydraulic pressure. The residual cakes contain still some oil but nearly half their weight consists of nitrogenous compounds. The following table gives the composition of a sample of cottonseed-meal:

While the above shows the composition of a single sample of the meal it should be remembered that there may be wide variations from this standard due either to natural composition or to different degrees of the extraction of the oil.

The composition of the ash is given below:

The cakes left after the expression of the oil from flaxseed and other oily seeds are also very rich in nitrogenous matters; but these residues are chiefly used for cattle-feeding and only the undigested portions of them pass into the manure. Cottonseed cake-meal is not so well suited for cattle-feeding as the others mentioned, because of the cholin and betaïn which it contains; often in sufficient quantities to render its use dangerous to young animals. The danger in feeding increases as the total quantity of the two bases and also as the relative quantity of cholin to betaïn, the former base being more poisonous than the latter. In a sample of the mixed bases prepared in this laboratory from cottonseed cake-meal the cholin amounted to 17.5 and the betaïn to 82.5 per cent of the whole.[125]

The nitrogen contained in these bases is also included in the total nitrogen found in the meal. The actual proteid value of the numbers obtained for nitrogen is therefore less than that obtained for the whole of the nitrogen by the quantity present as nitrogenous bases.

In the United States cottonseed cake-meal is used in large quantities as a direct fertilizer but not so extensively for mixing as some of the other sources of nitrogen. Its delicate yellow color serves to distinguish it at once from the other bodies used for similar purposes. No special mention need be made of other oil-cake residues. They are quite similar in their composition and uses, and manner of treatment and analysis to the cottonseed product.

147. Fish Scrap.—Certain species of fish, such as the menhaden, are valued more highly for their oil and refuse than for food purposes. But even where fish in large quantities are prepared for human food, there is a considerable quantity of waste matter which is valuable for fertilizing purposes. The residue of fish from which the fat and oil have been extracted, is dried and ground for fertilizing uses. The fish scrap thus obtained is used extensively, especially on the Atlantic border of the United States, for furnishing the nitrogenous ingredient in mixed fertilizers, and also for direct application to the fields. In fish flesh deprived of oil and water, the content of phosphoric acid is about two and one-half per cent, while the proteid matter may amount to three-quarters of the whole.[126]

The use of fish for fertilizing purposes is not new. As early as 1621 the settlers at Plymouth were taught to fertilize their maize fields by Squanto, an Indian. According to Goode, the value of nitrogen derived from the menhaden alone was two million dollars in 1875.[127] In 1878 it is estimated that 200,000 tons of these fish were captured between Cape Henry and the Bay of Fundy. The use of fish scrap for nitrogenous fertilizing has, since then, become an established industry, and the analyst may well examine his samples for this source of nitrogen when they are manufactured at points on the Atlantic coast, in proximity to great fishing centers.

148. Dried Blood and Tankage.—The blood and débris from abattoirs afford abundant sources of nitrogen in a form easily oxidized by the microorganisms of the soil. Blood is prepared for use by simple drying and grinding. The intestines, scraps, and fragments of flesh resulting from trimming and cutting, are placed in tanks and steamed under pressure to remove the fat. The residue is dried and ground, forming the tankage of commerce. Dried blood is richer in proteid matter than any other substance in common use for fertilizing purposes. When in a perfectly dry state, it may contain as much as fourteen per cent of nitrogen, equivalent to nearly eighty-eight per cent of proteid or albuminoid matter. Tankage is less rich in nitrogen than dried blood, but still contains enough to make it a highly desirable constituent of manures. Naturally, it would vary more in its nitrogen content than dried blood.

149. Horn, Hoof, and Hair.—These bodies, although quite rich in nitrogen, are not well suited to fertilizing purposes on account of the extreme slowness of their decomposition. Their presence, therefore, should be regarded in the nature of a fraud, because by the usual methods of analysis they show a high percentage of nitrogen, and therefore acquire a fictitious value. The relative value of the nitrogen in these bodies as compared with the more desirable forms, is given in paragraph 5.

150. Ammoniacal Nitrogen.—In ammonia compounds, nitrogen is used chiefly for fertilizing purposes as sulfate. The ideal nitrogenous fertilizer would be a combination of the ammoniacal and nitric nitrogen found in ammonium nitrate. The high cost of this substance excludes its use except for experimental purposes.

151. Nitrogen in Guanos.—The nitrogen in guanos may be found partly as organic, partly as ammoniacal, and partly as nitric nitrogen. The high manurial value of guanos and bat deposits in caves, is due not only to their phosphoric acid, but also to the fact that part of the nitrogen is immediately available, while a part becomes assimilable by nitrification during the growing season. The content of nitrogen in guanos is extremely variable, and depends largely on the climatic conditions to which the deposit has been subjected. The state in which it exists is also a variable one, but with a constant tendency to assume finally the nitric condition.

The well-known habits of birds in congregating in rookeries during the nights, and at certain seasons of the year, tend to bring into a common receptacle the nitrogenous matters which they have gathered and which are deposited in their excrement and in the decay of their bodies. The feathers of birds are particularly rich in nitrogen, and the nitrogenous content of the flesh of fowls is also high. The decay therefore, of remains of birds, especially if it take place largely excluded from the leaching of water, tends to accumulate vast deposits of nitrogenous matter. If the conditions in such deposits be favorable to the processes of nitrification, the whole of the nitrogen, or at least the larger part of it, which has been collected in this débris, becomes finally converted into nitric acid, and is found combined with appropriate bases as deposits of nitrates. The nitrates of the guano deposits, and of the deposits in caves, arise in this way. If these deposits be subject to moderate leaching, the nitrate may become infiltered into the surrounding soil, making it very rich in this form of nitrogen. The beds and surrounding soils of caves are often found highly impregnated with nitrates.

While for our purpose, deposits of nitrates only are to be considered which are of sufficient value to bear transportation, yet much interest attaches to the formation of nitrates in the soil even when they are not of commercial importance.

In many soils of tropical regions not subject to heavy rainfalls, the accumulation of these nitrates is very great. Müntz and Marcano[128] have investigated many of these soils, to which attention was called first by Humboldt and Boussingault. They state that these soils are incomparably more rich in nitrates than the most fertile soils of Europe. The samples which they examined were collected from different parts of Venezuela and from the valleys of the Orinoco, as well as on the shore of the Sea of Antilles. The nitrated soils are very abundant in this region of South America, where they cover large surfaces. Their composition is variable, but in all of them calcium carbonate and phosphate are met with, and organic nitrogenous material. The nitric acid is found always combined with lime. In some of the soils as high as thirty per cent of calcium nitrate have been found. Nitrification of organic material takes place very rapidly the year round in this tropical region. These nitrated soils are everywhere abundant around caves, as described by Humboldt, which serve as the refuge of birds and bats. The nitrogenous matters, which come from the decay of the remains of these animals, form true deposits of guano, which are gradually spread around, and which, in contact with the limestone and with access of air, suffer complete nitrification with the fixation of the nitric acid by the lime.

Large quantities of this guano are also due to the débris of insects, fragments of elytra, scales of the wings of butterflies, etc., which are brought together in those places by the millions of cubic meters. The nitrification, which takes place in these deposits, has been found to extend its products to a distance of several kilometers through the soil. In some places the quantity of calcium nitrate is so great in the soils that they are converted into a plastic paste by this deliquescent salt.

152. Nitric Nitrogen.—In its purer forms, and suited to manurial purposes, nitric acid exists in combination with sodium as a compound commonly known as Chile saltpeter.

The existence of these nitrate deposits has long been known.[129] The old Indian laws originally prohibited the collection of the salt, but nevertheless it was secretly collected and sold. Up to the year 1821, soda saltpeter was not known in Europe except as a laboratory product. About this time the naturalist, Mariano de Rivero, found on the Pacific coast, in the Province of Tarapacà, immense new deposits of the salt. Later the salt was found in equal abundance in the Territory of Antofagasta, and further to the south in the desert of Atacama, which forms the Department of Taltal.

At the present time the collection and export of saltpeter from Chile is a business of great importance. The largest export which has ever taken place in one year was in 1890, when the amount exported was 927,290,430 kilograms; of this quantity 642,506,985 kilograms were sent to England and 86,124,870 kilograms to the United States. Since that time the imports of this salt into the United States have largely increased.

According to Pissis[130] these deposits are of very ancient origin. This geologist is of the opinion that the nitrate deposits are the result of the decomposition of feldspathic rocks, the bases thus produced gradually becoming united with the nitric acid provided from the air.

According to the theory of Nöllner[131] the deposits are of more modern origin, and due to the decomposition of marine vegetation. Continuous solution of soils beneath the sea gives rise to the formation of great lakes of saturated water, in which occurs the development of much marine vegetation. On the evaporation of this water, due to geologic isolation, the decomposition of nitrogenous organic matter causes generation of nitric acid, which, coming in contact with the calcareous rocks, attacks them, forming calcium nitrate, which, in presence of sodium sulfate, gives rise to a double decomposition into sodium nitrate and calcium sulfate.

The fact that iodin is found in greater or less quantity in Chile saltpeter is one of the chief supports of this hypothesis of marine origin, inasmuch as iodin is always found in sea plants, and not in terrestrial plants. Further than this, it must be taken into consideration that these deposits of sodium nitrate contain neither shells nor fossils, nor do they contain any calcium phosphate. The theory, therefore, that they are due to animal origin is scarcely tenable.

Lately extensive nitrate deposits have been discovered in the U. S. of Columbia.[132] These deposits have been found extending over thirty square miles and vary in thickness from one to ten feet. The visible supply is estimated at 7,372,800,000 tons, containing from 1.0 to 13.5 percent of nitrate. The deposits consist of a mixture of sodium nitrate, sodium chlorid, calcium sulfate, aluminum sulfate, and insoluble silica. It is thought that the amount of these deposits will almost equal those in Chile and Peru.

METHODS OF ANALYSIS.

153. Classification of Methods.—In general there are three direct methods of determining the nitrogen content of fertilizers. First the nitrogen may be secured in a gaseous form and the volume thereof, under standard conditions, measured and the weight of nitrogen computed. This process is commonly known as the absolute method. Practically it has passed out of use in fertilizer work, or is practiced only as a check against new and untried methods, or on certain nitrogenous compounds which do not readily yield all their nitrogen by the other methods. The process, first perfected by Dumas, who has also given it his name, consists in the combustion of the nitrogenous body in an environment of copper oxid by which the nitrogen, by reason of its inertness, is left in a gaseous state after the oxidation of the other constituents; viz., carbon and hydrogen, originally present.

In the second class of methods the nitrogen is converted into ammonia which is absorbed by an excess of standard acid, the residue of which is determined by subsequent titration with a standard alkali. There are two distinct processes belonging to this class, in one of which ammonia is directly produced by dry combustion of an organic nitrogenous compound with an alkali, and in the other ammonium sulfate is produced by moist combustion with sulfuric acid, and the salt thus formed is subsequently distilled with an alkali, and the free ammonia thus formed estimated as above described. Nitric nitrogen may also be reduced to ammonia by nascent hydrogen either in an acid or alkaline solution as described in volume first.

In the third class of determinations is included the estimation of nitric nitrogen by colorimetric methods as described in the first volume. These processes have little practical value in connection with the analyses of commercial fertilizers, but find their chief use in the detection and estimation of extremely minute quantities of nitrites and nitrates. In the following paragraphs will be given the standard methods for the determination of nitrogen in practical work with fertilizing materials and fertilizers.

154. Official Methods.—The methods adopted by the Association of Official Agricultural Chemists have been developed by more than ten years of co-operative work on the part of the leading agricultural chemists of the United States. These methods should be strictly followed in all essential points by all analysts in cases where comparison with other data are concerned. Future experience will doubtless improve the processes both in respect of accuracy and simplicity, but it must be granted that, as at present practiced, they give essentially accurate results.

155. Volumetric Estimation by Combustion with Copper Oxid.—This classical method of analysis is based on the supposition that by the combustion of a substance containing nitrogen in copper oxid and conducting the products of the oxidation over red-hot copper oxid and metallic copper, all of the nitrogen present in whatever form will be obtained in a free state and can subsequently be measured as a gas. The air originally present in all parts of the apparatus must first be removed either by a mercury pump or by carbon dioxid or by both together, the residual carbon dioxid being absorbed by a solution of caustic alkali. Great delicacy of manipulation is necessary to secure a perfect vacuum and as a rule a small quantity of gas may be measured other than nitrogen so that the results of the analyses are often a trifle too high. The presence of another element associated with nitrogen, or the possible allotropic existence of that element, may also prove to be a disturbing factor in this long-practiced analytical process. For instance, if nitrogen be contaminated with another element, e. g., argon, of a greater density the commonly accepted weight of a liter of nitrogen is too great and tables of calculation based on that weight would give results too high.

First will be given the official method for this process, followed by a few simple variations thereof, as practiced in this laboratory.

156. The Official Volumetric Method.—This process may be used for nitrogen in any form of combination.[133]

The apparatus and reagents needed are as follows:

Combustion tube of best hard Bohemian glass, about sixty-six centimeters long and 12.7 millimeters internal diameter.

Azotometer of at least 100 cubic centimeters capacity, accurately calibrated.

Sprengel mercury air-pump.

Small paper scoop, easily made from stiff writing paper.

Coarse cupric oxid.—To be ignited and cooled before using.

Fine cupric oxid.—Prepared by pounding ordinary cupric oxid in a mortar.

Metallic copper.—Granulated copper, or fine copper gauze, reduced and cooled in a current of hydrogen.

Sodium bicarbonate.—Free from organic matter.

Caustic potash solution.—Make a supersaturated solution of caustic potash in hot water. When absorption of carbon dioxid, during the combustion, ceases to be prompt, the solution must be discarded.

Filling the tube.—Of ordinary commercial fertilizers take from one to two grams for analysis. In the case of highly nitrogenized substances the amount to be taken must be regulated by the amount of nitrogen estimated to be present. Fill the tube as follows: (1) About five centimeters of coarse cupric oxid: (2) Place on the small paper scoop enough of the fine cupric oxid to fill, after having been mixed with the substance to be analyzed, about ten centimeters of the tube; pour on this the substance, rinsing the watch-glass with a little of the fine oxid, and mix thoroughly with a spatula; pour into the tube, rinsing the scoop with a little fine oxid: (3) About thirty centimeters of coarse cupric oxid: (4) About seven centimeters of metallic copper: (5) About six centimeters of coarse cupric oxid (anterior layer): (6) A small plug of asbestos: (7) From eight-tenths to one gram of sodium bicarbonate: (8) A large, loose plug of asbestos; place the tube in the furnace, leaving about two and five-tenths centimeters of it projecting; connect with the pump by a rubber stopper smeared with glycerol, taking care to make the connection perfectly tight.

Operation.—Exhaust the air from the tube by means of the pump. When a vacuum has been obtained allow the flow of mercury to continue; light the gas under that part of the tube containing the metallic copper, the anterior layer of cupric oxid (see (5) above), and the sodium bicarbonate. As soon as the vacuum is destroyed and the apparatus filled with carbon dioxid, shut off the flow of mercury and at once introduce the delivery-tube of the pump into the receiving arm of the azotometer just below the surface of the mercury seal, so that the escaping bubbles will pass into the air and not into the tube, thus avoiding the useless saturation of the caustic potash solution.

Set the pump in motion and when the flow of carbon dioxid has very nearly or completely ceased, pass the delivery-tube down into the receiving arm, so that the bubbles will escape into the azotometer. Light the gas under the thirty centimeter layer of oxid, heat gently for a few moments to drive out any moisture that may be present, and bring to a red heat. Heat gradually the mixture of substance and oxid, lighting one jet at a time. Avoid a too rapid evolution of bubbles, which should be allowed to escape at the rate of about one per second or a little faster.

When the jets under the mixture have all been turned on, light the gas under the layer of oxid at the end of the tube. When the evolution of gas has ceased, turn out all the lights except those under the metallic copper and anterior layer of oxid, and allow to cool for a few moments. Exhaust with the pump and remove the azotometer before the flow of mercury is stopped. Break the connection of the tube with the pump, stop the flow of mercury, and extinguish the lights. Allow the azotometer to stand for at least an hour, or cool with a stream of water until a permanent volume and temperature have been reached.

Adjust accurately the level of the potash solution in the bulb to that in the azotometer; note the volume of gas, temperature, and height of barometer; make calculation as usual, or read results from tables.

156. Note on Official Volumetric Method.—The determination of nitrogen in its gaseous state by combustion with copper oxid, has practically gone out of use as an analytical method. The official chemists rarely use it even for control work on samples sent out for comparative analysis. The method recommended differs considerably from the process of Jenkins and Johnson, on which it is based. The only source of oxygen in the official method is in the copper oxid. Hence it is necessary that the oxid in immediate contact with the organic matter be in a sufficiently fine state of subdivision, and that the substance itself be very finely powdered and intimately mixed with the oxidizing material. Failure to attend to these precautions will be followed by an incomplete combustion and a consequent deficit in the volume of nitrogen obtained.

The copper oxid before using is ignited, and is best filled into the tube while still warm by means of a long pointed metal scoop, or other convenient method. The copper spiral, after use, is reduced at a red heat in a current of hydrogen, and may thus be used many times.

157. The Pump.—Any form of mercury pump which will secure a complete vacuum may be used. A most excellent one can be arranged in any laboratory at a very small expense. The pump used in this laboratory for many years answers every purpose, and costs practically nothing, being made out of old material not very valuable for other use.

The construction of the pump and its use in connection with the combustion tube will be clearly understood from the following description:

A glass bulb I is attached, by means of a heavy rubber tube carrying a screw clamp, to the glass tube A, having heavy walls and a small internal diameter, and being one meter or more in length. The tube A is continued in the form of a U, the two arms being joined by very heavy rubber tubing securely wired. The ends of the glass tubes in the rubber should be bent so that they come near together and form the bend of the U, the rubber simply holding them in place. This is better then to have the tube continuous, avoiding danger of breaking. A tee tube, T, made of the same kind of glass as A, is connected by one arm, a, with the manometer B, by a heavy rubber union well wired. The union is made perfectly air-tight by the tube filled with mercury held by a rubber stopper. The middle arm of the tee, a′, is expanded into a bulb, E, branching into two arms, one of which is connected with A and the other with the delivery-tube F, by the mercury-rubber unions, MM′, just described. The interior of the bulb E should be of such a shape as to allow each drop of mercury to fall at once into F without accumulating in large quantity and being discharged in mass. The third arm of the tee a″ is bent upwards at the end and passes into a mercury sealing tube, D, where it is connected by means of a rubber tube with the delivery-tube from the furnace. The flow of the mercury is regulated by the clamp C, and care should be taken that the supply does not get so low in I as to permit air bubbles to enter A. The manometer B dips into the tube of mercury H. A pump thus constructed is simple, flexible, and perfectly tight. The only part which needs to be specially made is the tee and the one in use here was blown in our own laboratory. The bent end of the delivery-tube F may also be united to the main tube by a rubber joint thus aiding in inserting it into the V-shaped nozzle of the azotometer.

The azotometer used is the one devised by Schiff and modified by Johnson and Jenkins.[134]

We prefer to get the V nozzles separately and join them to any good burette by a rubber tube. The water-jacket is not necessary, but the apparatus can be left exposed until it reaches room temperature.

Any form of mercury pump capable of securing a vacuum may be used, but the one just described is commended by simplicity, economy, effectiveness, and long use.

158. The Pump and Combustion Furnace.—The pump and combustion furnace, as used in the laboratory, are shown in Fig. 10. The pump is constructed as just described, and rests in a wooden tray which catches and holds any mercury which may be spilled. The furnace is placed under a hood which carries off the products of the burning lamps and the hot air. A well-ventilated hood is an important accessory to this process, especially when it is carried on in summer. A small mercury pneumatic trough catches the overflow from the pump and also serves to immerse the end of the delivery-tube during the exhaustion of the combustion tube.

The other details of the arrangement and connections have been sufficiently shown in the previous paragraph.

159. Volumetric Method in this Laboratory.—It has been found convenient here to vary slightly the method of the official chemists in the following respects: The tube used for the combustion is made of hard refractory glass, which will keep its shape at a high red heat. It is drawn out and sealed at one end after being well cleaned and dried. It should be about eighty centimeters in length and from twelve to fourteen millimeters in internal diameter. The relative lengths of the spaces occupied by the several contents of the tube are approximately as follows: Sodium bicarbonate, two; asbestos, three; coarse copper oxid, eight; fine copper oxid, containing sample, sixteen; coarse copper oxid, twenty-five; spiral copper gauze, ten to fifteen; copper oxid, eight; and asbestos plug, five centimeters, respectively.

The copper oxid should be heated for a considerable time to redness in a muffle with free access of air before using and the copper gauze be reduced to pure metallic copper in a current of hydrogen at a low red heat. The anterior layer of copper oxid serves to oxidize any hydrogen that may have been occluded by the copper. When a sample is burned containing all or a considerable part of the nitrogen as nitrates, the longer piece of copper gauze is used.

160. The Combustion.—The tube having been charged and connected with the pump it is first freed from air by running the pump until the mercury no longer rises in the manometer. The end of the tube containing the sodium bicarbonate is then gently heated so that the evolution of carbon dioxid will be at such a rate as to slowly depress the mercury in the manometer, but never fast enough to exceed the capacity of the pump to remove it. The lamp is extinguished under the sodium carbonate and the carbon dioxid completely removed by means of the pump. The delivery-tube is then connected with the azotometer, and the combustion tube carefully heated from the front end backwards, the copper gauze and coarse copper oxid being raised to a red heat before the part containing the sample is reached. When the nitrogen begins to come off, its flow should be so regulated by means of the lamps under the tube, as to be regular and not too rapid. From half an hour to an hour should be employed in completing the combustion. Since most samples of fertilizer contain organic matter, the nitrogen will be mixed with aqueous vapor and carbon dioxid. The former is condensed before reaching the azotometer, and the latter is absorbed by the potassium hydroxid. When the sample is wholly of a mineral nature it should be mixed with some pure sugar, about half a gram, before being placed in the tube. When bubbles of gas no longer come over, the heat should be carried back until there is a gradual evolution of carbon dioxid under the conditions above noted. Finally the gas is turned off and the pump kept in operation until the manometer again shows a perfect vacuum when the operation may be considered finished. In the manipulation our chief variation from the official method consists in connecting the combustion apparatus with the measuring tube before the heat is applied to the front end of the combustion tube. Any particles of the sample which may have stuck to the sides of the tube on filling will thus be subject to combustion and the gases produced measured. Where it is certain that no such adhesion has taken place it is somewhat safer on account of the possible presence of occluded gases to heat the front end of the tube before connecting the combustion apparatus with the azotometer.

161. Method of Johnson and Jenkins.—In the method of Johnson and Jenkins the principal variation from the process described consists in introducing into the combustion tube a source of oxygen whereby any difficultly combustible carbon may be easily oxidized and all the nitrogen be more certainly set free.[135] The potassium chlorate used for this purpose is placed in the posterior part of the tube, which is bent at slight angle to receive it. The sodium bicarbonate is placed in the anterior end of the tube. The combustion goes on as already described, and at its close the potassium chlorate is heated to evolve the oxygen. The free oxygen is absorbed by the reduced copper oxid, or consumed by the unburned carbon. Any excess of oxygen is recognized at once by its action on the copper spiral. As soon as this shows signs of oxidation the evolution of the gas is stopped. Care must be taken not to allow the oxygen to come off so rapidly as to escape entire absorption by the contents of the combustion tube. In such a case the nitrogen in the measuring tube would be contaminated.

It is rarely necessary in fertilizer analysis to have need of more oxygen than is contained in the copper oxid powder in contact with the sample during the progress of combustion.

162. Calculation of Results.—The nitrogen originally present in a definite weight of any substance having been obtained in a gaseous form its volume is read directly in the burette in which it is collected. This instrument may be of many forms but the essential feature of its construction is that it should be accurately calibrated; and the divisions so graduated as to permit of the reading of the volume accurately to a tenth of a cubic centimeter. For this purpose it is best that the internal diameter of the measuring tube be rather small so that at least each ten cubic centimeters occupies a space ten centimeters long. The volume occupied by any gas varies directly with the temperature and inversely with the pressure to which it is subjected. The quantity of aqueous vapor which a moist gas may contain is also a factor to be considered. Inasmuch as the nitrogen in the above process of analysis is collected over a strong solution of potassium hydroxid capable of practically keeping the gas in a dry state the tension of the aqueous vapor may be neglected.

163. Reading the Barometer.—Nearly all the barometers in use in this country have the scale divided in inches and the thermometers thereunto attached are graduated in Fahrenheit degrees. This is especially true of the barometers of the Weather Bureau which are the most reliable and most easy of access to analysts. It is not necessary to correct the reading of the barometer for altitude, but it is important to take account of the temperature at the time of observation. There is not space here to give minute directions for using a barometer. Such directions have been prepared by the Weather Bureau and those desiring it can get copies of the circular.[136]

The temperature of a barometer affects its accuracy in two ways. First the metal scale expands and contracts with changing temperatures: Second, the mercury expands and contracts also at a much greater rate than the scale. If a barometer tube hold thirty cubic inches of mercury the contents will be one ounce lighter at 80° F. than at 32° F. The true pressure of the air is therefore not shown by the observed height of the mercurial column unless the temperature of the scale and of the mercurial column be considered.

Tables of correction for temperature are computed by simple formulas based on the known coefficients of expansion of mercury and brass. For barometers with brass scales the following formula is used for making the correction:

In this formula t = temperature in degrees Fahrenheit and h = observed reading of the barometer in inches.

Unless extremely accurate work be required the correction for temperature is of very little importance in nitrogen determinations in fertilizers. Each instrument sent out by the Weather Bureau is accompanied by a special card of corrections therefor, but these are of small importance in fertilizer work. In order then to get the correct weight of the gas from its volume the reading of the thermometer and barometer at the time of measurement must be carefully noted. However, after the end of the combustion, the azotometer should be carried into another room which has not been affected by the combustion and allowed to stand until it has reached the room temperature.

Every true gas changes its volume under varying temperatures at the same rate and this rate is the coefficient of gaseous expansion. For one degree of temperature it amounts to 0.003665 of its volume. Representing the coefficient of expansion by K the volume of the gas as read by V, the volume desired at any temperature by V′, the temperature at which the volume is read by t and the desired temperature by t′, the change in volume may be calculated by the following formula:

V′ = V[1 + K(t′ - t)].

Example.—Let the volume of nitrogen obtained by combustion be thirty-five cubic centimeters, and the temperature of observation 22°. What would be the volume of the gas at 0°?

Making the proper substitutions in the formula the equation is reduced to the form below:

V′ = 35[1 + 0.003665(0°-22°)]

or V′ = 35(1 - 0.08063) = 32.18.

Thirty-five cubic centimeters of nitrogen therefore measured at 22° become 32.18 cubic centimeters when measured at 0°.

When gases are to be converted into weight after having been determined by volume, their volume at 0° must first be determined; but this volume must also be calculated to some definite barometric pressure. By common consent this pressure has been taken as that exerted by a column of mercury 760 millimeters in height. Since the volume of a gas is inversely proportional to the pressure to which it is subjected, the calculation is made according to that simple formula. Let the reading of the barometer, at the time of taking the volume of gas, be H, and any other pressure desired H′. Then we have the general formula: