THE PHYSICAL PROPERTIES OF THE PROTEINS 17 



strengths of even 80-85 per cent, on the addition of small quantities 

 of salts. The albumin of horse serum can be redissolved under 

 these circumstances by the addition of acids, bases or salts. If the 

 horse serum be not dialysed before precipitation with alcohol, re- 

 solution in alcohol only takes place on the addition of acid or 

 fairly strong alkalies. The dissolved serum-albumin does not lose 

 its original properties. Egg-albumin, even when dialysed, acquires 

 the property of being redissolved in alcohol wholly, by addition of 

 bases, partially, by addition of acids, and not at all, by addition of 

 salts. Nothing is known of the causes of these phenomena. 



Many proteins are also soluble in urea solutions (Ramsden) and 

 also in organic bases. This solubility is due probably to their acidic 

 nature. Globulins, caseinogen, acid- and alkali-albumin and even 

 heat-coagulated proteins dissolve in a saturated aqueous solution of 

 pure urea. Dry gelatin dissolves at room temperature till a 40 per 

 cent, solution is obtained. If the urea be removed by dialysis, the 

 gelatin sets to a jelly again. The presence of urea, furthermore, pre- 

 vents coagulation of solutions. 



SECTION IV. SEPARATION OF PROTEINS FROM SOLUTION BY 

 PRECIPITANTS OTHER THAN SALTS. 



Michaelis and Rona have recently suggested a new precipitant for 

 proteins. When an alcoholic solution of gum-mastic is added to 

 water an emulsion is formed, which, on addition of salts, is de-emul- 

 sified with the formation of a flocculent precipitate. If proteins be 

 present in the emulsion colloidal particles will be obtained, consisting 

 both of the mastic and the protein. If the former be in sufficient 

 excess the whole of the latter will be carried down, when sufficient 

 electrolyte (e.g., hydrochloric acid) has been added to de-emulsify the 

 solution. The action is irreversible, in that the protein cannot be 

 dissolved out from the precipitate by water. On treatment, however, 

 with organic solvents, the mastic can be dissolved ; if ether be used 

 as a solvent hardly any nitrogenous matter goes into solution ; with 

 chloroform or alcohol, on the other hand, very appreciable quantities 

 of nitrogenous matter are dissolved. The method has the disadvan- 

 tage when applied to the separation of many proteins, in that the 

 latter, when the attempt is made to recover them by means of ether, 

 are rendered insoluble (coagulated), and thus cannot be obtained in 

 their original form. The mastic can also partially precipitate pro- 

 teose ; in the case of a commercial peptone obtained from fibrin, Rona 

 and Michaelis found about 88 per cent, of the nitrogen in the filtrate, 

 the remainder being precipitated by the mastic. They suggest the 

 employment of the method for the separation of proteoses. In this 

 case the mastic can be separated from the precipitated proteose 

 without altering this substance. In solutions containing coagulable 

 protein, such as serum, the method may be of use in estimating the 

 non-protein constituents, such as urea, nitrogenous extractives and 

 sugar, which are not carried down by the mastic precipitate. 



This method is, so far, new. Owing to the irreversibility it does 

 not seem as if it will be capable of such general application as the 

 salt-precipitation method for the isolation of proteins ; its chief use is 

 for freeing solutions from proteins for the subsequent examination of 



