98 



Mr. W. B. Hardy. On the Mechanism of 



not to be constant for a given mixture. The internal phase formed 

 droplets which were large or small according to whether the mixture 

 was cooled slowly or rapidly. Thus with a mixture containing 

 13 '5 grammes gelatine per 100 c.c. the droplets (of solid solution) were 

 very regularly 3/x in diameter when about 20 c.c. was allowed to cool 

 slowly in air. Cooled rapidly, however, in an ether spray, the drop- 

 lets were so minute as barely to be visible with a magnification of 

 400 diameters. The effect of the rate of cooling is the same when 

 mixtures with a large gelatine content are used, and when, therefore, 

 the internal phase is a fluid solution at ordinary temperatures. When 

 cooling is very rapid the droplets are excessively minute ; when it is 

 slow they may be as large as 10/x in diameter (gelatine 36'5 per cent, 

 of the mixture). One can therefore make the general statement that 

 the more slowly the division into two phases occurs the smaller and less curved 

 is the surface of separation. 



The effect upon the structure of the rate at which a fresh condition 

 is imposed upon the system is manifested in a very striking way when 

 an already formed gel is cooled. The experiments upon the effect of 

 temperature on the composition of the two phases in the case of the 

 hydrogel of agar show that when heat is added to or taken away 

 from the system the balance of the phases is altered, water, and 

 perhaps agar, passing from the one to the other. It might be ex- 

 pected that this would take place solely by the passage of material 

 across the surface which separates the two phases. The study of the 

 ternary mixtures, however, makes it clear that a new approximate 

 equilibrium may be reached in two distinct ways. 



When a portion of the hydrogel of gelatine-water-alcohol is cooled 

 slowly from 16° to, say, 3° or 4°, one can see with the micro- 

 scope no change beyond an alteration in the size of the droplets 

 already present, that is to say, the fresh (approximate) equilibrium 

 is attained by exchange across the surface which separates the phases. 

 But if the cooling is rapid, say a fall of 10° in a few minutes, a second- 

 ary system of small droplets appears. 



In all the mixtures which I examined these were formed in the 

 external phase. Thus, when the concentration of gelatine in the 

 whole mass was low, it was the fluid phase which underwent a division 

 into secondary phases ; when it was high, it was the solid phase. To 

 put this fact in a general way, one can say that when the hydrogel is 

 exposed to a rapid fall of temperature the phase which lies on the convex side 

 of the surface of separation undergoes division into two secondary phases/^ 

 When the temperature is again allowed to rise these secondary phases 

 fuse before there is any obvious change in the relation of the primary 

 phases. 



* The formation of tne secondary phases therefore occurs in that one of the 

 primary phases which is under the lower hydrostatic pressure. 



