716 PROTEIDS. [Apr. 



by the action of gastric juice in an acid, or of pancreatic juice in an 

 alkaline medium. 



All proteids in solutions are precipitated by an excess of strong 

 alcohol. If the precipitant be rapidly removed they are again soluble 

 in water, but if the precipitated proteids are subjected for some time to 

 the action of the alcohol they are, with the exception of peptones, 

 coagulated and lose their solubility. It appears however that the 

 proteids contained in the aleurone-grains of plants are exceedingly 

 resistant to this coagulating action of alcohol ! . 



It seems scarcely necessary to point out the distinction in the use of the word 

 ' coagulation ' as applied to blood- or muscle-plasma on the one hand and to the 

 action of heat and alcohol upon proteids on the other. The difference is obvious 

 when it is remembered that in the first case the coagulation leads to the formation 

 of fibrin (Class iv.), or myosin (Class m.), and that these bodies may then further 

 be coagulated by heat or alcohol as described above. 



CLASS VI. Peptones. 



Very soluble in water, and not precipitated from their aqueous 

 solutions by the addition of acids or alkalis, or by boiling. Insoluble 

 in alcohol, they are precipitated with difficulty by this reagent, and aro 

 unchanged in the process ; they differ from all other proteids in not being 

 coagulated by prolonged exposure to alcohol. They are not precipitated 

 by cupric sulphate, ferric chloride, or, except in the instances to be 

 mentioned presently, by potassic ferrocyamde, and acetic acid. In 

 these points they differ from most other proteids. On the other hand, 

 precipitation is caused by chlorine, iodine, tannin, mercuric chloride, 

 nitrates of mercury and silver, and both acetates of lead ; also by bile- 

 acids in an acid solution. In common with all proteids, these bodies 

 possess a specific laevo-rotatory power over polarised light; but they 

 differ from all other proteids in the fact that boiling produces no change 

 in the amount of rotation. 



A solution of peptones, mixed with a strong solution of caustic 

 potash, gives, on the addition of a mare trace of cupric sulphate, a pink 

 colour. An excess of the cupric salt gives a violet colour, which 

 deepens in tint on boiling, in fact the ordinary proteid reaction. Other 

 proteids simply give the violet colour. But the most characteristic 

 feature of peptones is their relatively great diffusibility, a property which 

 they alone, of all the proteids, may be said to possess, since all other forms 

 of proteids pass through membranes with the greatest difficulty, if at all. 



The diffusibility of peptones is however absolutely small as compared with that 

 of crystalline bodies such as sodic chloride; in fact solutions of peptones may be 

 freed from salts by dialysis, a process employed in their preparation. 



1 See Vines, JL of PhysioL Vol. in. p. 108. 



