238 Biological Chemistry. 



the salts of heavy metals. The composition of the pre- 

 cipitate will, in the last-named case, vary with the relative 

 amounts of heavy metal and protein in solution, and the 

 phenomenon is a somewhat complex one, which need not 

 be discussed in detail here. 



(6) Proteins are precipitated by the reagents which 

 are used for precipitation of the more complex basic sub- 

 stances, known as the alkaloids (p. 226). The chief are 

 phosphotungstic and phosphomolybdic acid, potassium* 

 mercuric iodide and potassium-bismuth iodide, tannic acid 

 and picric acid. The last-named is often used in a solu- 

 tion containing 1 per cent, picric and 1 per cent, citric 

 acids, known generally as Esbach's reagent. 



(7) A very important property of the proteins is that 

 they can be precipitated, as already mentioned, from 

 aqueous solutions by certain salts other than those of the 

 heavy metals. This precipitation is, unlike coagulation, a 

 reversible process that is, the precipitate produced will 

 redissolve in salt-free water, or, in the case of globulins, in 

 water containing a relatively small amount of salt. The 

 salts generally employed for protein precipitation are 

 sodium chloride, and the sulphates of ammonium, mag- 

 nesium, and zinc. The degrees of saturation by a given salt, 

 in which precipitation for any given protein (and certain 

 derivatives of proteins such as proteoses) begins and is 

 completed, is characteristic for each individual member of 

 the class. As already stated, a globulin, such as serum 

 globulin, can be precipitated from solution when the latter 

 is saturated with magnesium sulphate. If an albumin is 

 present at the same time, this, for the most part, remains 

 in solution, and a partial separation of the two proteins 

 can be accomplished.* 



* It cannot be pretended that a complete separation of proteins, when they 

 exist together in solution, can be accomplished in this way. The globulin 

 precipitate would carry down with it a certain amount of the albumin. 



