172 



One slioiild naturally expect that the course of melting 

 be diiforent t'l'om tliat of freezing;', since, in frozen tissue, 

 the two constituent phases are se])arated, one of tlicni, 

 water, hciu^' in tlic iiilci'cclhilai's, the ol hei', partly dehy- 

 drated i)r()t()plasm, remaining- within the cell; a frozen 

 tissue to be melted is, therefore, a quite different system 

 from an intact tissue to be frozen. 



More investigation on melting curves of tissues, in par- 

 ticuhir of tissues wliich survive a ])artial freezing and still 

 possess semipermeable membranes, would throw some 

 light on several questions related to osmosis during freez- 

 ing and thawing and, in general, on the forces binding- 

 water in living protoplasm. A comparison between melt- 

 ing curves of living and of dead tissues might also prove 

 illuminating. 



B. ALTERATIONS OBSERVED IN THAWED MATERIAL 



The fundamental alteration caused in aqueous solutions 

 and suspensions, in aqueous colloids and in protoplasm by 

 freezing is the separation of ice. If, when ice is thawed, 

 the system can be restored to the condition in which it was 

 before freezing, in other words, if the phase separation is 

 reversible, the material might be unaltered. In this sense, 

 the true solutions are, at least on first approximation, un- 

 altered by freezing. Whether or not in suspensions, in 

 colloids and in protoplasm the phase separation, or any 

 other alteration, is reversible upon thawing, is the subject 

 of the following inquiry. 



1. Suspensions and Colloids. Vogel (1820) observed 

 that starch paste loses its adhesive properties after being 

 frozen. This is one of the oldest reports concerning a 

 permanent change induced by freezing in a colloid. 



According to Goeppert (1830), when the milky plants 

 Euphorhia and Ficiis are frozen, the milk is irreversibly 

 transformed into a watery fluid. 



Ljubavin (J. Buss. Phys.-Chem. Soc, 21, 397, 1889) was 

 the first, according to Lottermoser (1907), to show that a 

 hydrosol can be converted into a gel by freezing. He also 

 pointed out tlie influence of electrolytes on that conversion. 



