174 



Agar-agar and golatiii solutions sliowcd, for ([uilc a long 

 time after thawing, a donhlc i)liase sti-ncturc consisting 

 of t'lunij)s of jelly and of a dilute li(iuid. Albumin solu- 

 tions stayed jH'i-mancntly lurhid aftci' treatment at -70° 

 and -ISO but not after being frozen at -10 . Platinum 

 and gold were precipitated from their colloidal solutions 

 by a freezing at -70°. Solutions of arsenic trisulfide and 

 of antimony trisultide also ))i-ecipitated. A dilute solu- 

 tion of iron hydroxide {li(iu<)r fcrri (Ualifsafi) did not pre- 

 cipitate, but pi-esented an inci'eased Tyndall etfect. A 

 solution of sodium silicate remained entii-ely unaltered. 

 In silver preiKirations, such as protargol, kollargol and 

 lysargin, the silver agglomerated into clumps, irregularly 

 distributed in the ice; but, after thawing, the preparations 

 were just the same as before freezing (a protective effect 

 of albumin is thought to be resi)onsible for the reversible 

 action in these preparations). 



The authors think that, in all the above solutions, solute 

 particles are brought together during congelation and 

 that, after thawing, they may or may not be dispersed 

 again, depending on the nature of the substance. Be- 

 sides, some colloids would be irreversibly dehydrated l)y 

 freezing. 



According to Bruni (1909), colloidal isinglass, frozen at 

 -20° or at -80°, formed, after thawing, a gel which could 

 not be distinguished from the original one. But colloidal 

 silicic acid, treated in the same manner, consisted, after 

 thawing, of two phases, one of which was pure water and 

 the other, a precipitate of amorphous leaflets of hydrated 

 silicic acid. 



In 1911, Fischer reviewed the investigations of most of 

 the previous workers, including those which he had made 

 himself with various collaborators, and he concluded that 

 colloids show an extreme variation in resistance to cold. 

 In general, the changes which they undergo on freezing 

 are reversible but, for some colloids, at some detinite tem- 

 ])ei'atures, iirevei'sible changes take ])lace. The tempera- 

 tures at which these iri-evei'sible pi-ocesses occur are some- 



