398 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXV. 



precipitate with proteids which is turned brick-red on boiling. This 

 reaction and the preceding (xanthroproteic) depend on the presence 

 in proteids of aromatic radicles. 



(3) Copper sulphate (Rose's or Piotrowski's) test. A trace of copper 

 sulphate and excess of strong caustic potash give with most proteids 

 a violet solution. Proteoses and peptones, however, give a rose-red 

 colour instead; this same colour is given by the substan'ce called 

 liuret ; hence the test is generally called the biuret reaction. This 

 name does not imply that biuret is present in proteid; but both 

 proteid and biuret give the reaction because they possess a common 

 radicle, probably CONH. 



Biuret is formed by heating solid urea ; ammonia passes off and leaves biuret 

 thus : 



2CON 2 H 4 - NH 3 = C 2 2 N 3 H 5 . 



[Urea.] [Ammonia.] [Biuret.] 



(4) Adamkiewicz reaction (Hopkins' modification). When a solu- 

 tion of proteid is added to a dilute solution of glyoxylic acid, and 

 then excess of sulphuric acid is added, an intense violet colour is 

 obtained. 



Precipitants of Proteids. Solutions of most proteids are pre- 

 cipitated by: 



1. Strong acids like nitric acid. 



2. Picric acid. 



3. Acetic acid and potassium ferrocyanide. 



4. Acetic acid and excess of a neutral salt like sodium sulphate ; 

 when these are boiled with the proteid solution. 



5. Salts of the heavy metals like copper sulphate, mercuric 

 chloride, lead acetate, silver nitrate, etc. 



6. Tannin. 



7. Alcohol. 



8. Saturation with certain neutral salts such as ammonium 

 sulphate. 



It is necessary that the words coagulation and precipitation should 

 in connection with proteids be carefully distinguished. The term 

 coagulation is used when an insoluble proteid (coagulated proteid) is 

 formed from a soluble one. This may occur : 



1. When a proteid is heated heat coagulation ; 



2. Under the influence of a ferment ; for instance, when a curd is 

 formed in milk by rennet or a clot in shed blood by the fibrin ferment 

 -ferment coagulation ; 



3. When an insoluble precipitate is produced by the addition of 

 certain reagents (nitric acid, picric acid, tannin, etc.). 



There are, however, other precipitants of proteids in which the 

 precipitate formed is readily soluble in suitable reagents like saline 

 solutions, and the proteid continues to show its typical reactions. 



