122 THE PROTEINS AND THE AMINO-ACIDS 



upon dehydration of the protein and the formation of complex insoluble 

 anhydrides, Precipitation-reactions, which depend upon the formation 

 of insoluble compounds with the precipitating-agents employed, and 

 Color-reactions which depend upon chemical interaction with the 

 reagents employed, resulting in the production of distinctive colors. 

 The most important of these reactions are the following: 



COAGULATION-REACTIONS. 



1. Heat. Heat applied to solutions acidified with ace_tic acid. If 

 mineral acids are employed, compounds of the protein with the acid 

 may be formed which are incoagulable by heat. 



2. Alcohol. Alcohol added to neutral or acid solutions. 



3. Concentrated Neutral Salts. Concentrated neutral salts, particu- 

 larly ammonium sulphate or magnesium sulphate. In ajcidified solutions 

 concentrated sodium chloride or sodium sulphate are also coagulants of 

 protein. 



4. Strong Mineral Acids. Upon the ability of the strong_mineral 

 acids to coagulate proteins depends Heller's Test for protein in urine. 

 The suspected sample of urine is placed in a test tube and concen- 

 trated nitric acid is allowed to flow into the bottom of the tube from 

 a pipette. At the junction of the two fluids a white ring of coagulated 

 protein is formed. 



Precipitation Reactions. 1. The Salts of Heavy Metals, such as 

 cupric sulphate, lead acetate, mercuric chloride, silver nitrate, etc., 

 form insoluble compounds with proteins. In the presence of excess 

 of the reagent, the precipitate which at first forms not infrequently 

 redissolves. 



. 2. The so-called Alkaloidal Reagents, such as phosphotungstic or 

 phosphomolybdic acids, tannic acid, potassium mercuric iodide, picric 

 acid, trichloracetic acid, phenol and salicyl-sulphonic acid. Other 

 reagents which similarly precipitate proteins are metaphosphoric acid, 

 nucleic acids, chondroitin-sulphuric acid and taurocholic acid. Potassium 

 ferrocyanide and acetic acid yield an insoluble compound of the protein 

 with hydroferrocyanic acid. 



Color -reactions. 1. Millon's Reaction. A solution of mercury in 

 strong nitric acid yields a mixture of mercuric and mercurous nitrates 

 dissolved in a mixture of nitric and nitrous acids. If this reagent 

 be added to a protein solution, a precipitate is produced which turns 

 brick-red on heating. 



2. The Xanthoproteic Reaction. On adding strong nitric acid to 

 protein solutions and heating to boiling, a pale yellow solution or 

 coagulum results. On rendering the mixture alkaline with ammonia 

 it becomes orange-yellow. 



3. The Hopkins-Cole Reaction. A solution of glyoxylic acid, formed 

 by acting upon a concentrated solution of oxalic acid with sodium 

 amalgam, is added to the protein solution in a test tube, and sulphuric 



