756 A MANUAL OF VETERINARY PHYSIOLOGY 



with a protein solution, the mixture turns red, and the same colour 

 is imparted to a precipitate should it occur. This reaction is 

 supposed to be due to the presence of tyrosine in the molecular 

 grouping, and is therefore not given when this is absent. 



Piotrowski's Reaction. — To the solution of protein an excess 

 of strong solution of caustic soda is added, and one or two drops 

 of a i per cent, solution of copper sulphate. A purple colour results 

 in the presence of protein, and a rose-red colour in the presence of 

 proteoses and peptone. The essential feature is that the colour 

 must be a purple, either red or blue. When protein in any form is 

 absent, the solution remains blue. The term biuret is applied to this 

 test, as a similar reaction is given with biuret, which is formed 

 by heating urea. 



Adamkiewicz's Reaction. — A mixture of one volume of con- 

 centrated sulphuric acid and two volumes of glacial acetic acid, 

 when added to a solution of protein, produces a reddish-violet colour 



Fig. 260. — Albumin Crystals from Horse-Serum (Gurber). 



and slight fluorescence. The reaction is due to the presence of 

 tryptophane in the molecule. 



The precipitation of proteins is brought about on the addition of 

 mineral acids, such as nitric ; salts of the heavy metals — for instance, 

 acetate of lead or mercuric chloride — and by boiling ; also by excess 

 of alcohol; neutral salts of the alkalies — i.e., sodium chloride and 

 ammonium sulphate — to the point of saturation, and in many other 

 ways. Halliburton draws attention to the necessity for distin- 

 guishing between coagulation and precipitation. For instance, 

 a protein, on heating or on adding nitric acid, is coagulated ; the 

 precipitate is insoluble, but precipitates obtained by the addition 

 of such substances as ammonium sulphate are soluble, and can 

 readily be sent back into solution. To these the term ' precipitate ' 

 especially applies. The circumstances under which such precipitates 

 are obtained have been mentioned under the head of Albumins 

 and Globulins. 



