ACTIVATION OF UNFERTILIZED STARFISH EGGS. 



295 



tine, agar, soaps, lipoids and other hydrophilous colloids. The 

 relations of temperature to this process show in fact a close 

 resemblance to those described above for the activation-process. 

 One striking peculiarity of melting and gelation is that both 

 processes take place gradually; when {e. g.) a gelatine sol is 

 brought below the gelation-temperature and the conditions are 

 then kept constant, the actual solidification takes place only 

 after the lapse of a considerable period of time. The time re- 

 quired to reach the gelation-stage decreases rapidly as tempera- 

 ture is lowered; thus Levites found that a gelatine sol kept un- 

 disturbed at 26° took 26 hours to gelatinize, at 25° only 1 1 hours. ^ 

 The first observable change in the solution is an increase in 

 viscosity; this continues until the system sets; the setting repre- 

 sents the end-stage of the whole process, whose course can thus 

 be traced by successive viscosity-determinations. Gelation is 

 thus equivalent to a progressive increase in viscosity to a final 

 stage at which the ordinary fluid mobility is lost.^ It is found 

 that above a certain temperature the viscosity of the hydrosol 

 undergoes no change with time; but if the temperature is 

 lowered a critical point is eventually reached below which the 

 viscosity undergoes steady increase (at a rate dependent on 

 temperature, presence of salts, reaction) until gelation occurs. 

 The rate of this increase in viscosity (i. e., of the gelation-process), 

 Ar]lAt, shows a high temperature-coefficient. With a i per cent, 

 gelatine solution Schroeder^ obtained the following values for the 

 viscosity at 21°, 24.8°, and 31° at different intervals after bringing 

 the warm gelatine solution to the temperature of observation: 



Interval. 



5 mm 

 10 min 

 15 min 

 30 min 

 60 min 



Thus while at 31° the viscosity undergoes no change with time, 



1 Levites, Kolloid-Zeitschrift, 1907, Vol. 2, p. 211. 



2 Cf. Schroeder, Zeitschriftfilr physikalische Chemie, 1903, Vol. 45, p. 75; Levites: 

 loc. cit., p. 209; Freundlich, "Kapillarchemie," 1909, pp. 416 ff. 



3 Schroeder, loc. cit., p. 88. 



