THE PROTEIN SUBSTANCES. 79 



writers consider them as psendo bases, and hold that they 

 become actual bases only through peculiar transformations 

 effected by the addition of acids. But careful studies of 

 Osborne have shown that they act as true bases and unite 

 directly with acids, often in simple proportions. Many of the 

 substances which we commonly call pure proteins are in 

 reality salts of the true bases. These salts are decomposed 

 by addition of weak alkali and in presence of litmus a neutral 

 reaction is not secured until after the addition of a certain 

 volume of the alkali solution. Usually a still further volume 

 of alkali must be added to produce neutrality with phenol- 

 phthalein. 



In this manner several of the proteins seem to unite with 

 acid in the proportion of one molecule of protein to two mole- 

 cules of a monohydrogen acid. Because of the very great 

 molecular weight of the protein such determinations are not 

 easy, however. 



The presence of sulphur in the proteins was shown by a test referred 

 to some pages back. Investigations have shown that sulphur is present 

 in at least two kinds of combinations in the protein complex; there must 

 be at least two sulphur atoms in the molecule. Some of the sulphur is 

 easily separated by hot alkali solutions, while the rest of it is not. No 

 part of this element appears to be combined in oxidized form, that is, in 

 the condition of a sulphite or sulphate. The sulphur compounds which 

 have been obtained in protein decomposition are such as may be derived 

 from a breaking down of the cystin group. It has been shown that cystin 

 gives up its sulphur very slowly to boiling alkali, and then only in part, 

 as sulphide. 



The general reactions and characteristics of the protein 

 bodies having been discussed, a brief description of the more 

 important individual substances will now follow. 



TRUE OR NATIVE ALBUMINS. 



In the scheme of classification given some pages back the 

 true or native albumins have the first place. The best known 

 representatives of the protein group are included here. 



