LIVING SUBSTANCE 107 



determined, which are characteristic for proteids and in doubtful 

 cases make known their presence. What chemical transformation 

 the proteid molecule undergoes in these tests is of course prac- 

 tically unknown. The best-known tests are the following, any one 

 of which alone is riot sufficient to prove with absolute certainty the 

 presence of albumin : 



1. The xanthoproteic test : a solution of proteid is coloured 



yellow by boiling with nitric acid ; by the addition of 

 ammonia the colour changes to orange. 



2. The biuret test : if a solution of proteid be made alkaline 



by causic potash or soda, it takes on in the cold, by the 

 addition of a drop of cupric sulphate solution, a clear 

 violet colour. 



3. Milton's test : coagulated proteid, boiled for a time with 



a solution of mercuric nitrate, and a little nitrous acid, 

 becomes rose-red. 



4 The hydrochloric acid test : boiling with concentrated 

 hydrochloric acid dissolves coagulated proteids and 

 colours the clear liquid violet. 



5. The potassium ferrocyanide test : a solution of proteid to 



which acetic acid has been added shows by the addition 

 of a solution of potassium ferrocyanide a white cloudiness. 



6. The iodine test : the addition of tincture of iodine, or a 



solution of iodine in potassium iodide, serves as a good 

 microscopic method for recognising proteids ; the clot 

 becomes yellowish-brown. 



Besides these tests, a great number of others have been sug- 

 gested by different investigators, but they fail in individual cases. 



According to their different solubilities in water, three groups may 

 be distinguished among the simple proteids viz., the albumins, 

 the globulins, and the vitellins. 1 



The albumins are directly soluble in pure water. To them be- 

 long egg-albumin, which forms the great mass of the white of eggs ; 

 serum-albumin, an albuminous body contained in blood-serum ; 

 muscle-albumin, the proteid of muscle-cells, soluble in water ; and 

 plant-albumin, which is dissolved in the sap of plant-cells. 



The globulins are soluble in water only when it contains neu- 

 tral salts, but in less quantity than in saturation. If a solution of 

 globulin be saturated with salts, the globulin is precipitated in a 

 flocculent mass a phenomenon that is termed " salting out." The 

 globulin is likewise precipitated if the solution be wholly freed 

 from salts by diffusion in a dialyzer. To the globulins belong 

 serum-globulin, which is dissolved in the blood-serum ; fibrinogen, 

 also a proteid of blood, which coagulates spontaneously into flakes 

 and threads of fibrin when the blood is allowed to stand outside 

 the blood-vessels ; myosin, the globulin of muscle, which likewise 



1 Neumeister ('93}. 



