SPECIFIC GRAVITY 349 



Chick and Martin (21) have compared the densities of proteins 

 in the dissolved and in the dry conditions and find invariably an 

 increase in density of the dissolved protein corresponding to the 

 volume-contraction which accompanies the passage of proteins 

 into solution. Thus dry casein suspended in benzol has a specific 

 gravity of 1.318, while a 7.85 per cent solution of casein in dilute 

 sodium hydroxide has a specific gravity of 1.0240, whence the 

 density of the dissolved sodium caseinate = 1.42, while correcting 

 for the effect due to sodium hydroxide, the dissolved casein itself 

 has a density of 1.39. 



The following table summarizes the results obtained with 

 various proteins: 



The more dilute a solution of sodium caseinate, the greater its 

 apparent density was found to be. This was not the case in solu- 

 tions of serum globulin, or albumin. 



12. The Magnetic Properties of the Proteins. — It was shown 

 in 1845 by Faraday (29) and later by Pliicker (83) that haemo- 

 globin contains iron ; it is, nevertheless, like the other constituents 

 of living tissues, decidedly diamagnetic. This observation has 

 been confirmed in an extended series of investigations by Gamgee 

 (32), who finds that not only oxyhsemoglobin but also CO-hsemo- 

 globin and methsemoglobin are strongly diamagnetic bodies. The 

 iron-containing radical, hsematin, and its derivative hsemin are, 

 on the contrary, as might be expected from their content of iron, 

 strongly magnetic bodies. 



Benedicenti (7) (8) finds that the presence of protein inhibits 

 the lowering of the diamagnetic constant of water by the addition 

 of dissolved ferric chloride or finely pulverized iron. 



13. *'The Gold-Number" of Proteins. — It was found by 

 Zsigmondy (135) that colloidal gold is precipitated from its solution 

 (or suspension) by the addition of sodium chloride, but that this 

 precipitation can be prevented by the addition to the solution of 

 many other colloids, among which may be reckoned the proteins. 



