386 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Albumin. 



Solutions of albumin coagulate on heating at temperatures varying 

 from 50 to 70. The exact temperature depends upon the amount 

 and kind of salt present ; in the presence of thiocyanates heat coagu- 

 lation does not occur even at the boiling-point. The transformation 

 is irreversible. 



Solutions of albumin are precipitated by high concentrations of 

 electrolytes (Na 2 SO 4 , (NH 4 ) 2 SO 4 , MgSO 4 ). These precipitates redis- 

 solve in water ; the transformation is reversible. 



The precipitates formed by CaCl 2 , SrCl 2 , BaCl 2 become insoluble 

 on standing, whilst the precipitates formed by solutions of heavy 

 metals are insoluble. 



Caseinogen, Gum- Arabic. 



These sols do not coagulate on heating and do not form gels. 

 The solutions are simply more or less viscous at different temperatures. 



Cellulose, Nitrocellulose. 



These sols form coherent gels when the solvent is removed by 

 evaporation or by washing out with water. 



Transition of Emulsoids to true Solutions. 



Some substances form emulsoid sols in one solvent, but true 

 solutions in another solvent 



E.g. soap in water is an emulsoid sol, in alcohol a true solution ; 

 tannin in glacial acetic acid is a true solution, in water an emulsoid sol. 



There are many differences amongst the dye-stuffs ; eosin resembles 

 a true solution ; fuchsin forms an emulsoid sol. 



Nature of Emulsoids. 



Emulsoids possess many of the properties of emulsions, especially 

 high viscosity, and they show many differences from the suspensoids. 

 Their behaviour can only be explained on the assumption that they 

 are systems of two liquid phases, i.e. as systems consisting of dilute 

 solutions of a colloid containing droplets or globules of more concen- 

 trated solution. 



They differ from emulsions in the ease in which the solvent may 

 pass from one phase into the other. Gelatin sol is a continuous liquid 

 phase containing droplets of higher concentration : gelatin jelly is a 

 continuous solid phase containing droplets of dilute liquid. In dis- 

 solving gelatin, sol formation takes place by imbibition of water and 

 swelling ; there is disintegration of the original system. 



The effect of salts upon the coagulation of albumin and upon jelly 

 formation is to affect the distribution of the solvent between the two 

 phases. They act by altering the compressibility of water. Solution 

 of an emulsoid generally occurs with contraction. 



