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.