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: