CHEMICAL COMPOSITION OF THE HUMAN BODY 29 



and the reaction of the solution. In strongly acid or alkaline 

 solutions, coagulation does not occur. 



The coagulation does not produce any real change in the nature 

 of the proteid ; probably only an anhydrid condensation or poly- 

 merization takes place. Coagulated proteid contains less ash than 

 non-coagulated proteid; hence by coagulation a part of the 

 mineral constituents of the proteid is split off. 



2. Many proteids are precipitated by alcoliol. If the 

 action of the alcohol is allowed to continue, the precipitated 

 proteid is coagulated. 



3. Nearly all proteids are precipitated by saturating their 

 solutions with neutral salts (sodium chloride, magnesium, 

 sodium, and especially ammonium sulphate). An acid 

 reaction favors this precipitation. 



The precipitation by salts is first of all due to the fact that the 

 salts deprive the proteid of the solvent. Still, chemical action 

 is not excluded. In using ammonium sulphate, for example, 

 ammonia is set free, for the sulphuric acid unites with the precipi- 

 tated proteid and at last causes a splitting up of the proteid. 



By precipitation by salts, proteids can be obtained in crystalline 

 form. Proteids cannot, like other substances, be crystallized by 

 mere evaporation of the water which holds them in- solution, for, 

 in proportion as the water is evaporated, the proteids are precipi- 

 tated and form a solid pellicle at the surface of the water. Hence 

 the solution does not reach the condition pf supersaturation 

 necessary for crystallization. This can, however, be -obtained by 

 using salt as a precipitant, whereby the concentration of the salt 

 is gradually increased either by the careful addition of the salt to 

 the solution or by evaporation. For this purpose, sodium or 

 ammonium sulphate is best. 



4. Proteids are precipitated by concentrated mineral acids, 

 especially by nitric acid (Heller's test). Metaphosphoric 

 acid readily precipitates proteids, while the orthophosphoric 

 acid does so with difficulty. 



5. Proteids are precipitated by the salts of the heavy 

 metals, copper sulphate, ferric chloride, neutral and basic 

 lead acetate, platinum chloride, corrosive sublimate in hydro- 

 chloric acid solution. In these, the heavy metals unite with 

 the proteids as a weak acid, forming a compound insoluble 

 in water. 



