426 CHEMICAL DISCOVERY AND INVENTION 



2. Secondary Protein Derivatives. 

 a. Proteoses. 

 6. Peptones. 

 c. Peptides. 



The simple proteins and the conjugated proteins are all sub- 

 stances which are supposed to exist in the tissues and juices of 

 animals and vegetables. They are separated by taking advantage 

 of their solubility or insolubility in saline solutions, such as 

 aqueous ammonium sulphate or sodium chloride or in alcohol of 

 different strengths. They are all substances which present more 

 or less decidedly an " amphoteric " character, that is they have 

 the power of uniting either with acids or with bases owing to the 

 presence in them of carboxylic groups, -CO- OH, or amino groups, 

 -NH 2 . The molecular weight is not known accurately, but is 

 certainly very great. The composition of serum albumin, which 

 may be regarded as typical, and so far as percentages of the 

 elements are concerned, is closely similar to other albumins. It 

 is as follows : 1 



Carbon ..... 53-08 per cent. 



Hydrogen 7-10 



Nitrogen 15-93 



Sulphur 1-90 



Oxygen 21-99 



From various considerations a formula has been calculated, 



C 4 5oH 7 20-^1 lekgOj 40 



which corresponds to a molecular weight 10,176. Very little 

 importance can be attached to such expressions, they only serve 

 to indicate a recognition of the highly complicated character of 

 the molecule. The albumins are for the most part colloid 

 substances, though many of them have been observed naturally 

 or obtained by laboratory processes in a crystalline state. 



Many of these compounds are coagulated by heat and are 

 precipitated from solution in water, in the form familiar enough in 

 white of egg, as a kind of network of films and fibres which are 

 not reconvertible into the soluble form. The albumins are also 

 precipitated by the addition to the aqueous solution of the strong 



1 Michel, quoted in Mann's Proteids, p. 327. 



