OFFICIAL METHODS 323 



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 produces a 

 solution of the sample too highly colored to be used as above, 

 alcohol of 80 per cent, strength may be substituted. The colora- 

 tion 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 exam- 

 ination 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 albuminoid nitrogen the sample is 

 exhausted with water and heated with soda-lime, which gives 

 rise to ammonia which may be detected as described above. 



282. 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. 

 JKrequently, however, especially in cases of finely ground mixed 

 goods, the microscope must be employed to determine the charac- 

 ter 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 expe- 

 rience or by the circumstances connected with any particular in- 

 stance. In such cases the general appearance, odor, and consis- 

 tence of the sample may afford valuable indications which will 

 aid in discovering the origin of the nitrogenous materials. 



283. Official Methods. The methods adopted by the Asso- 

 ciation of Official Agricultural Chemists have been developed 

 by more than 20 years of co-operative work on the part of the 

 leading agricultural chemists of the United States. These meth- 

 ods should be strictly followed in all essential points by all analysts 



