173 



Bredig {Ztsch. f. angeiv. (Item., 954, 1898) described a 

 precipitation of platinum and of othci- metals fi-om col- 

 loidal solutions, after melting'. 



Lottermoser (1907 and 1908) confirmed these results 

 and, in particular, the fact that the lower the electrolyte 

 content of a sol is, the easier it acquires the property of 

 precipitating after thawing (although there are excep- 

 tions). Under ordinary conditions, hydrosols with high 

 electrolyte content, such as silicic acid, undergo no change 

 by being frozen and thawed; but when their electrolyte 

 content has been reduced by dialysis they can be precipi- 

 tated. The changes in consistency were concomitant with 

 changes in electric conductivity. The author considers 

 precipitation a consequence of the withdrawal of water 

 during freezing. The fine structure of the gel, he suggests, 

 is destroyed by the increase in volume of solidifying water. 

 The particles of the colloid set free in that breaking down 

 of the structure are wedged between the expanding ice 

 crystals where they are transformed into leaflets which, 

 after thawing, precipitate to the bottom of the vessel. 



Bobertag, Feist and Fischer (1908) studied the action 

 of congelation at -10°, -70° and -180° on a large number 

 of substances belonging to various groups : true solutions 

 of dyes, such as eosin, safranin, etc. ; partly colloidal solu- 

 tions, fuchsin, methyl violet, etc., colloidal solutions of 

 Congo red, benzopurpurin, etc.; colloidal solutions of 

 tannin, gums, starch, agar-agar, gelatin, albumin and 

 hemoglobin; colloidal solutions of metal and metal com- 

 pounds. Some dyes were partly decolorized after thaw- 

 ing, but usually the previous color was resumed later, 

 although in some cases it took 2 days for the restitution of 

 the original condition. Tannin solutions precipitated 

 above the freezing point; on thawing, the precipitate fell 

 to the bottom of the container and, on rewarming, it dis- 

 solved again ; this is the ordinary behaviour of a solute of 

 which the solubility decreases at lower temperatures. 

 Gum, starch and hemoglobin solutions were turbid immedi- 

 ately after thawing, but cleared up entirely afterwards. 



