304 PEACTICAL PHYSIOLOGY 



(D) Sodium Sulphate possesses at 30 C. the same protein precipitating 

 powers as ammonium sulphate. It is of great advantage when it is 

 desired to estimate the amount of protein in any fluid. By precipitat- 

 ing with sodium sulphate and determining the total nitrogen in the 

 precipitate by Kjeldahl's method (see Urine) the amount of protein is 

 found by multiplying by 6*25. 



(E) Zinc Sulphate has also been used for salting-out purposes. 



VIII. Coagulants of Proteins. A coagulum differs from a precipitate 

 in that it is no longer soluble in its original solvent ; in other words, 

 its physical or chemical nature has undergone some change. Such is 

 the case in the coagulation of protein by heat. Other coagulants of 

 protein are : mechanical agitation, mineral acids and salts, and other 

 acids such as tannic, picric, etc. 



EXPERIMENT XIII. Violently shake some egg-white solution with 

 sand. Strings of coagulated protein are deposited. 



EXPERIMENT XIV. To some egg-white solution add gently some 

 strong HN0 3 . A white precipitate appears, which is insoluble on 

 heating (cf. Proteoses). 



EXPERIMENT XV. Acidulate some egg-white solution strongly with 

 acetic acid, then add strong potassium ferrocyanide a whitish yellow 

 precipitate. 



EXPERIMENT XVI. Add picric acid a white precipitate. Many 

 other acids, such as phosphomolybdic, phosphotungstic, trichloracetic 

 and salicyl sulphonic are used to precipitate proteins. 



Alcohol precipitates all proteins. At first it forms a precipitate; but 

 if the action be prolonged this changes to a coagulum. Peptone . and 

 fibrin ferment (thrombin) take longer to undergo this change ; advan- 

 tage is taken of this to separate these bodies from other proteins (see 

 Preparation of Fibrin Ferment, 336). 



CHAPTER V. 

 PROTEINS CONTINUED. 



THE CLASSIFICATION OF PROTEINS. 



THE following classification has now been adopted for proteins. It is 

 based partly upon the results of chemical investigation, partly upon 

 .such properties as solubility, " salting out," etc. It cannot, therefore, 

 be regarded as complete. 



