Protoplasmic StreaTuing — Relation to Gel Structure 153 



sodium oleate and other purely thixotropic gels, no change of volume 

 occurs when gelation or solation occurs at a fixed temperature, and 

 the shift of equilibrium which is induced by periods of shaking or 

 rest, is isothermal. The second type, exemplified by gelation or agar 

 gels, displays a small decrease of volume with setting, and heat is 

 evolved in the process. Finally, the third type, which is represented 

 by colloidal aqueous solutions of methylcellulose, shows a small 

 increase of volume and an absorption of heat during the setting 

 process. Thus it is found that gels of the second type tend to set 

 upon cooling, whereas those of the third type undergo solation as 

 the temperature falls. 



From the principle of Le Chatelier, it appears certain that hydro- 

 static pressure would favor solation in the third type of gel, gelation 

 in the second type, and little or no change in the first. And the 

 experimental evidence in this regard, although scanty, clearly sub- 

 stantiates the predictions. Thus Marsland and Brown ('39) have 

 found that gelatin sols, which are known to belong in the second 

 category, undergo marked gelation at pressures in the physiological 

 range, whereas gel preparations of myosin extracted from rabbit 

 muscle are solated to a degree which corresponds, at least in quali- 

 tative fashion, with the solational effects of pressure on protoplasmic 

 gels. 



In all probability, therefore, the various protoplasmic gels which 

 have proved so susceptible to solation under pressure, belong to 

 Freundlich's group III. This viewpoint is borne out by the truly 

 remarkable regularity of the effect in different systems — for in 

 every case which has been studied, each unit increment of pressure 

 produces the same proportionate loss in the tensile properties of the 

 gel which is being tested. If these changes were being effected 

 indirectly through the medium of other chemical reactions, such a 

 degree of regularity would scarcely be expected in so many different 

 kinds of cell. Furthermore, it has been observed (Costello, '34) 

 that the cortical gel of the Arbacia egg becomes less "viscous" as the 

 temperature falls, a behavior which would not be expected unless 

 this gel is of the third type. 



The common component upon which the pressure acts in the 

 various cells probably will prove to be one or more groups of 

 protein substances which is capable of forming gels of the nature 

 of type III. And in this connection it is interesting to note that 

 Mirsky ('36) has extracted from the Arbacia egg (a cell which very 

 plainly shows the characteristic pressure-solation effect) a protein 



