1184 PROTEIDS. 



2. Add carefully a few drops of pure nitric acid; a pre- 

 cipitate is formed if the acid is not in excess, which disappears 

 on warming and comes again on cooling. 



3. Add acetic acid, avoiding all excess, and then a trace of 

 potassium ferrocyanide ; a precipitate is formed which disap- 

 pears on warming and reappears on cooling. 



4. On the addition of caustic soda in excess and a trace of 

 sulphate of copper the ordinary biuret reaction is obtained. 

 This reaction distinguishes hemialbumose from other soluble 

 proteids, with the exception of peptones. 



Hemialbumose has so far been spoken of as if it were one 

 uniform substance only. More recent research has shown 

 that four closely allied but distinct forms of proteid have to be 

 dealt with under the name hemialbumose, so that this word has 

 now acquired a historic rather than an actual designative sig- 

 nificance. These substances are distinguished by the following 

 names and reactions. 1. Protoalbumose. Soluble in water and 

 precipitable by saturation with sodium chloride. 2. Heteroal- 

 bumose. Insoluble in water but soluble in dilute (5 — 10 p.c.) 

 solutions of sodium chloride, from which, like protoalbumose, 

 it is precipitable by saturation with the salt. 3. Dysalbumose. 

 Insoluble in either water or solutions of sodium chloride. It 

 appears to be merely a modified form of heteroalbumose resulting 

 from the prolonged action of water and neutral salts or of being 

 kept dry. It may be readily reconverted into heteroalbumose 

 by solution in dilute hydrochloric acid (-2 p.c.) or caustic soda 

 (1 p.c.) and reprecipitation by neutralizing either of these 

 solutions. 4. D enter oalbumose. Soluble in water and not pre- 

 cipitable from this solution by saturation with sodium chloride 

 unless an acid be added at the same time. 



Solutions of protoalbumose and heteroalbumose yield precipi- 

 tates directly on the addition of nitric acid; deuteroalbumose 

 does so only in presence of sodium chloride. The latter is not 

 precipitated by sulphate of copper; the two former are. Hetero- 

 albumose is not precipitable by mercuric chloride, whereas 

 proto- and deutero-albumose are so. 



The albumoses are slightly diffusible, but less so than the 

 peptones. 



The peptones. Since the albumoses and peptones must be 

 regarded as a series of progressive products of the hydrolytic 

 decomposition of proteids, it is difficult to decide the exact point 

 at which the former pass into the latter, or in other words to 

 obtain a distinctive criterion for a true peptone. Hence in all 

 probability the various substances which have at earlier dates 

 been described as peptones have consisted to some extent, if not 

 largely, of a mixture of true peptones with variable quantities 

 of albumoses. Of late years it has become customary to dis- 

 criminate between the two classes of substances by reference 



