36 TEXT-BOOK OF PHYSIOLOGY. 



in strong acids and alkalies. All are insoluble in alcohol and 

 ether. 



3. Coagulability. Under the influence of heat and various acids 



and animal ferments, the proteids readily pass from the soluble 

 liquid state to the insoluble solid state, attended by a permanent 

 alteration in their chemic composition. To this change the 

 term coagulation has been given. The various proteids not 

 only coagulate at different temperatures, but with different 

 chemic reagents distinctive features which permit not only of 

 their detection, but separation. Proteids are capable of pre- 

 cipitation without losing their solubility by ammonium sulphate, 

 sodium chlorid, and magnesium sulphate. 



4. Fermentability. In the presence of specific microorganisms 



bacteria the proteids, owing to their complexity and instability, 

 are prone to undergo disintegration and reduction to simpler 

 compounds. This decomposition or putrefaction occurs most 

 readily when the conditions most favorable to the growth of 

 bacteria are present viz., a temperature varying from 25 C. 

 to 40 C., moisture, and oxygen. The intermediate as well as 

 the terminal products of the decomposition of the proteids are 

 numerous, and vary with the composition of the proteid and 

 "the specific physiologic action of the bacteria. Among the 

 intermediate products is a series of alkaloid bodies, some of 

 which possess marked toxic properties, know r n as ptomains or 

 toxins. The toxic symptoms which frequently follow the inges- 

 tion of foods in various stages of putrefaction are to be attributed 

 to these compounds. The terminal products are represented 

 by hydrogen sulphid, ammonia, carbon dioxid, fats, phosphates, 

 nitrates, etc. 



Color Tests for Proteids. When proteids are present in solu- 

 tion, they may be detected by the following color reactions viz. : 



1. Xanthoproteic. The solution is boiled with nitric acid for several 



minutes, when the proteid assumes a light yellow color. After 

 the solution has cooled, the addition of ammonia changes the 

 color to an orange or amber-red. 



2. The rose-red reaction. The solution is boiled with acid nitrate 



of mercury (Millon's reagent) for a few minutes, when the 

 coagulated proteid turns a purple-red color. 



3. The blue- violet reaction. A few drops of copper sulphate solution 



are first added to the proteid solution, and then an excess of 

 sodium hydroxid. A blue- violet color is produced, which deepens 

 somewhat on heating, but no further change ensues. 



