Chemical Constituents of the Animal Body. 239 



Again, a globulin is precipitated from its aqueous 

 solution when sufficient ammonium sulphate is present to 

 half-saturate it, whereas an albumin is precipitated only 

 from the completely saturated solution. The degrees of 

 saturation in which precipitation begins and ends will 

 vary slightly with the concentration of the protein. By 

 taking a standard concentration for example, 1 or 2 per 

 cent. it is possible to compare the precipitation limits of 

 two proteins; such limits serve to characterize the pro- 

 tein. This determination may be carried out in the 

 following manner : Into a series of test-tubes 2 c.c. of 

 a protein solution are diluted to 10 c.c. with varying 

 amounts of saturated salt solution and water. The filtrates 

 from the precipitate are then tested with one or two 

 drops of the saturated salt solution. The lowest dilution 

 of salt to cause precipitation, and the lowest dilution the 

 filtrate from which gives no precipitate on further addition 

 of salt, are noted. These two dilutions give the pre- 

 cipitation limits for any given salt. The salt precipitation 

 of proteins has been discussed in some detail, as this 

 process, imperfect as it is, affords the only means of sepa- 

 rating two proteins which exist together in one solution, 

 as fractional crystallization or distillation is in this case 

 quite inapplicable. 



COLOUR REACTIONS OF THE PROTEINS. 



The various colour reactions are not yielded by all 

 proteins. They are due to certain groups contained in 

 the molecule. 



(1) Nitric acid (sp. gr. 1- 2 1*25) imparts a bright 

 yellow colour to solid proteins ; the same colour is pro- 

 duced when solutions of proteins are boiled for some time 

 with strong acids. The reaction, known as the " xantho- 

 proteic " reaction, is probably due to the presence of a 



