vin THE GENERAL PHYSICAL PROPERTIES OF ALBUMINS 273 



1. Setting of Colloidal Solutions. + 



This question has been investigated by Hardy, 1 who calls jellies 

 which on heating become fluid, and on cooling revert to the solid 

 state, ' reversible jellies.' When colloidal matter occurs in the state of 

 a gel, which means a mixture of a fluid and a solid, then " it may consist 

 of a solid mass containing spherical fluid droplets, or of solid droplets, 

 which by hanging one to the other form a framework, in the spaces of 

 which fluid is held. These terotypes present important mechanical 

 peculiarities. 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 the open solid 

 framework therefore specially manifest that property of spontaneous 

 shrinkage to which Graham applied the term ' synaeresis.' " 2 



The author believes that the setting of gelatine which occurs on 

 cooling is in every respect comparable to the firming which molten 

 alloys of metals undergo when they are cooled. What attracts mole- 

 cules 3 of the same kind to one another when the temperature begins to 

 fall is the circumstance that they possess the same surface-tension ; 

 or with other words that they cannot develop amongst themselves 

 a difference of potential, which according to the author's view is 

 essential for the diffusion of one substance into another one, as ex- 

 plained on p. 255. As, further, a solution containing an electrolyte 

 becomes a better conductor when it is heated, and as this increase in 

 conductivity is not due to an additional electrolytic dissociation of 

 salt-molecules, it must be due to an increase in the mobility of the 

 already existing ions. We find, therefore, in a cooling salt solution an 

 increasing rigidity of the ions analogous to the increasing viscosity of 

 a cooling gelatine solution. 



When a gelatine solution cools, we have, therefore, firstly an aggre- 

 gation of the gelatine molecule on the one hand, and of the water 

 molecule on the other hand, because each species of molecule possesses 

 one surface tension, and secondly a ' setting ' owing to the firming of 

 each individual molecule. 



As examples of a partial setting when the temperature is raised 



1 Hardy, Journ. of Physiol. 24. 172 (1899). 



2 For further information see author, Physiological Histology, p. 49. 



3 The expression molecule is meant to include every kind of particle from au electron 

 to the largest aggregate of atoms capable of forming a definite compound. 



