18 HUMAN PHYSIOLOGY. 



2. Solubility. Some of the proteids are soluble in water, others in 

 solutions of the neutral salts of varying degrees of concentration, 

 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 differ- 

 ent temperatures, but with different chemic reagents distinctive 

 features which permit not only of their detection, but separation. 

 Proteids are capable of precipitation without losing their solubility 

 by ammonium sulphate, sodium chlorid and magnesium sulphate. 



4. Fermentability. In the presence of specific microorganisms bac- 

 teria the proteids, owing to their complexity and instability, are 

 prone to undergo disintegration and reduction to simpler com- 

 pounds. 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, 

 known as ptomains. The toxic symptoms which frequently follow 

 the ingestion of foods in various stages of putrefaction are to be 

 attributed to these compounds. The terminal products are repre- 

 sented by hydrogen sulphid, ammonia, carbon dioxid, fats, phos- 

 phates, nitrates, etc. 



Color Tests for Proteids. When proteids are present in solution, 

 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 



