NITROGENOUS COMPOUNDS 71 



selected and re-elaborated in a different proportion ; any group 

 or portion which does not enter into the composition of the 

 new compound is simply split off and rejected. 



Meta-proteins, Proteoses, and Peptones. These products 

 always resemble the parent protein more or less closely in 

 composition and general properties. They differ from it, and 

 from each other chiefly in regard to those physical properties 

 of colloids which depend on molecular complexity, e.g. solubility, 

 diffusibility, etc. 



The proteoses are not coagulated by heat, but they are 

 precipitated by alcohol, and are floculated by ammonium 

 sulphate. They react with mercuric chloride and salts of 

 other heavy metals, forming insoluble albuminates, are pre- 

 cipitated by ferrocyanic acid and other alkaloid reagents, and 

 they give the biuret and xantho-proteic reactions. They are 

 distinguished as primary and secondary pruteoses according 

 as they resemble more closely the parent protein or the 

 peptone into which they are ultimately transformed. The dis- 

 tinction between the secondary proteoses and the peptones is, 

 in some cases, purely arbitrary. Those which are precipitated 

 by cupric salts are classed as primary proteoses, and those 

 which are not, as secondary. This reaction may be used as a 

 means of separating the former from the latter. 



The various kinds of proteose are distinguished as albumose, 

 globulose, caseose, etc., according to the type (p. 72) of protein 

 from which they are derived. 



Peptones contain no sulphur. They are not coagulated by 

 heat, and are not salted out by ammonium sulphate. They 

 are not precipitated by alcohol, and do not form insoluble 

 albuminates with salts of heavy metals. They are, however, 

 precipitated by some of the alkaloid reagents, and they give 

 the biuret and xantho proteic reactions. The peptones are 

 simpler than the proteoses, i.e. they have a lower molecular 

 weight, and they are more soluble and diffuse more rapidly. 



When proteins are peptonised, it is always found that one 

 part is more resistant to the action of the ferment than the 

 other. The former yields glycocine, phenylalanine, and 

 pyrrolidine-carboxyllic acid. The latter contains tyrosine and 



