100 



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



fluid is held. These two types present important mechanical pecu- 

 liarities. The former is firm and elastic, and it maintains its structural 

 integrity even under high pressure. The latter is much more brittle, 

 and manifests a tendency to spontaneous shrinking, which is due to a 

 continuous increase in the surface of contact or possibly union between 

 droplet and droplet. These gels with an open solid framework there- 

 fore specially manifest that property of spontaneous shrinkage to which 

 Graham applied the term " synseresis." 



In the building of a hydrogel of the second type two distinct events 

 occur. The first is the separation of droplets, which rapidly become 

 solid ; the second is the linking of these droplets together to a pattern 

 so that they build a framework throughout the fluid phase. The first 

 is the separation of a homogeneous mass into two phases ; the second is 

 a phenomenon akin to the grouping of particles which are suspended in 

 a fluid. It is probable that these two events are not directly connected 

 with one another. 



Binary Mixtures (A gar-water). 



I know of no binary reversible system in which the optical characters 

 of the two phases differ sufficiently to permit of direct microscopical 

 investigation of the surface of separation. It is, however, easy to prove 

 that in such a system as agar and water the property of gel building is 

 dependent upon the appearance of two phases. 



The agar which was used was prepared from commercial agar as 

 follows. The strips were suspended in a large volume of distilled water 

 for twenty-four hours ; the water was then drained off, and a large part 

 of the water absorbed by the strips was squeezed out by a powerful 

 press. The strips were again suspended in distilled water, and again 

 drained and squeezed after forty-eight hours. This washing with 

 distilled water was continued for some weeks. The strips were then 

 melted and the hydrosol filtered, and the filtrate allowed to set. The 

 clear hydrogel so obtained was sliced and suspended for a further period 

 of weeks in many changes, of distilled water. In this way a colourless 

 gel was obtained free from all foreign diff'usible bodies. It was not 

 found necessary to take precautions against micro-organisms. With 

 the removal of the salts the agar ceased to afi'ord them a suitable 

 nidus. 



Effects of Pressure upon the Hydrogel of Agar. 



When gels containing 1 to 3 per cent, solids* are broken up and 

 slightly squeezed by hand a fluid exudes. In order to collect this fluid 

 a screw press was made use of. The gel was cut into pieces, which 

 were wrapped in fine cotton canvas which had been completely freed 



* By this is meant 1 to 3 grammes per 100 grammes. 



