1 6 HUMAN PHYSIOLOGY. 



proteins are amorphous, but vary in consistence from the liquid to the 

 solid state. The colloid character of the proteins permits of their sepa- 

 ration and purification from crystalloid diffusible compounds by the process 

 of dialysis. 



2. Solubility. Some of the proteins 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 proteins 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 proteins not only coagulate at different temperatures, but 

 with different chemic reagents distinctive features which permit not only 

 of their detection, but separation. Proteins are capable of precipitation 

 without losing their solubility by ammonium sulphate, sodium chlorid 

 and magnesium sulphate. 



4. Fermentability. In the presence of specific microorganisms bacteria 

 the proteins, 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 proteins are 

 numerous, and vary with the composition of the protein and the specific 

 physiologic action of the bacteria. Among the intermediate products 

 is a series of alkaloid bodies, some of which possess marked toxic prop- 

 erties, 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 represented by hydrogen 

 sulphid, ammonia, carbon dioxid, fats, phosphates, nitrates, etc. 



Color Tests for Proteins. When proteins 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 min 

 utes, when the protein assumes a light yellow color. After the solution 

 has cooled, the addition of ammonia changes the color to an orange or am- 

 berred. 



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

 cury (Millon's reagent) for a few minutes, when the coagulated protein 

 turns a purple-red color. 



