68 PHYSIOLOGICAL CHEMISTRY. 



Ex. The simplest coagulation may be shown py boiling a dilute white 

 of egg solution. As long as it is perfectly neutral coagulation follows at 

 once, but in presence of alkali acid must be added to the point of neu- 

 trality. This behavior finds practical application in the ordinary test for 

 serum albumin as it occurs pathologically in urine. The precipitate may 

 be redissolved only by some digestion or chemical process which produces 

 a new substance. 



Coagulation by Reagents. By the addition of certain 

 chemicals many of the protein compounds are easily thrown 

 into the coagulated condition. Some of the most character- 

 istic reactions in this direction are shown by simple experi- 

 ments with mineral acids, salts of heavy metals and the 

 so-called alkaloid reagents. 



Ex. Among the acids which bring about coagulation, nitric acid is the 

 most certain in its action and is commonly used in practical cases where 

 it is desired to recognize a small amount of serum or egg albumin in solu- 

 tion. The test may be made by adding about a cubic centimeter of strong 

 nitric acid to four or five cubic centimeters of white of egg solution and 

 warming. Coagulation follows at once. With a very dilute albumin 

 solution the substance separates in flakes, while with a strong solution a 

 stiff, jelly-like mass may result. The test is a common one in urine analy- 

 sis, but must be conducted with certain precautions. 



Ex. Solutions of most protein substances are precipitated by addition 

 of alcohol in excess. This may be shown by mixing white of egg solution 

 with strong alcohol, the latter being added gradually until the maximum 

 of precipitate is obtained. With dilute alcohol precipitates are usually 

 not secured, and besides the alcohol precipitation is usually not a perma- 

 nent coagulation as in the above case with the acid. 



Ex. Precipitation by Salts. Some of the salts of heavy metals give 

 characteristic precipitates with protein solutions. The behavior may be illus- 

 trated by adding to dilute egg albumin solution small amounts of solution 

 of mercuric chloride, lead acetate, copper sulphate or ferric chloride. The 

 reagents must not be added in excess, as in some cases this causes a re- 

 solution of the precipitate. Similar reactions may be obtained with solu- 

 tions of most of the heavy metals, but the salts mentioned are often used 

 in practice. The behavior of mercuric chloride or corrosive sublimate as 

 an active disinfecting agent depends on its property of coagulating the 

 protein matter of the pathogenic bacteria, to destroy which it is used. 

 The precipitates may be formed in neutral, acid or alkaline solution as a 

 rule, and chemically may be regarded as salts of the metals used as pre- 

 cipitants. 



Ex. Precipitation by Alkaloid Reagents. In acid solution the protein 

 bodies are very generally precipitated by addition of solutions of picric 

 acid, tannic acid, potassium-mercuric iodide, phosphomolybdic acid and 



