ELEMENTARY CHEMICAL PHYSIOLOGY 197 



(B) Magnesium Sulphate. 



EXPERIMENT. Fully saturate (i.e. add crystals) the solution of 

 egg-white with MgS0 4 . A precipitate of globulin results. Filter. 

 Prove by heat coagulation and by fully saturating with (NH 4 ) 2 SO 4 

 that protein (albumin) is left in the filtrate. Magnesium sulphate 

 in full saturation precipitates globulins, but not albumins. 



(C) Sodium Chloride, Ammonium Chloride. These salts resemble 

 magnesium sulphate in their " salting out " properties. 



(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 precipitating 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. 



IX. 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 I. Violently shake some egg-white solution with 

 sand. Strings of coagulated protein are deposited. 



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

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

 heating (cf. Proteoses). 



EXPERIMENT III. Acidulate some egg-white solution strongly 

 with acetic acid, then add strong potassium ferrocyanide a whitish 

 yellow precipitate. 



EXPERIMENT IV. Add picric acid a white precipitate. Many 

 other acids, such as phosphomolybdic, phosphotungstic, trichlor- 

 acetic 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 ; 

 advantage is taken of this to separate these bodies from other 

 proteins. 



CHAPTER III 

 PROTEINS Continued 



As regards the various groups of proteins they have all more or 

 less special properties. 



Protamines. These are usually found in the roe or milt of fish 

 as salts of nucleic acid. They are strongly basic substances which 

 readily turn litmus blue and take up carbon dioxide from the air. 

 They are soluble in water and are not coagulated on heating. As 

 a rule the only colour reaction given is the biuret. On decom- 



