220 Organic Constitucnts of Solls [Jan. 



when a red color will be formed which does not disappear on dilu- 

 tion. Tyrosine gives a similar color but the two cannot be confused 

 inasmtich as tyrosine crystallizes in needles that are nearly insoluble 

 in water. 



The following concluding Statements are quoted from pages 14, 

 15 and 16 of Bull. No. 80, U. S. Bureau of Soils (see foot-note 3) : 



" The amount of a substance obtained may be so small that extreme 

 purification is out of the question, and therefore in such cases, where 

 distinct crystalline form or characteristic tests are not available, the 

 Identification becomes uncertain, as neither melting point nor analysis 

 can be made. . . . 



" Experience has shown, therefore, that for the best results in search- 

 ing for a number of Compounds not less than 100 pounds of soil should 

 be used, unless the soil be very high in organic matter, or a specific test 

 applied, or the experimenter exceptionally experienced in the Isolation. 



" The scheme is an adaptation and coördination of the methods of 

 isolating from dififerent soils the single Compounds dealt with here. In 

 application of the scheme to different soils it has been found that for 

 each soil some few details in one part or other of the scheme must be 

 more or less modified. This modification is made necessary by the 

 fact that the organic material in different soils is really different, so 

 that the particular Compound occurs under different associations, which 

 often render a change in the method of isolation, identification, or 

 purification absolutely necessary. The experience and judgment of the 

 investigator must determine what modification to make. When a sub- 

 stance is isolated and is not identical with the soil constitutent already 

 found at this place in the scheme, it may nevertheless belong to the 

 same class of Compounds as the constituent already identified. In this 

 manner new Compounds may be discovered in the course of such an 

 investigation. Some of the filtrates and precipitates in the scheme are 

 discarded because as yet they have yielded no definite Compounds. How- 

 ever, these portions may be examined for such Compounds as the in- 

 vestigator sees fit. . , . 



" The portions designated for the final isolation and identification 

 of the various Compounds will almost always be contaminated with 

 coloring matter, resinous material, and other substances precipitated by 

 the metallic salts or soluble in the same solvents, and such foreign ma- 

 terial must, of course, be eliminated by careful reprecipitation or by 

 repeated Solution. The metallic salts selected for this investigation as 



