308 L. V. HEILBRUNN. 



protoplasm, it has been known that the living substance is vis- 

 cous. Like all other viscous colloidal substances, no doubt proto- 

 plasm can undergo sudden marked increases or decreases in 

 viscosity. These changes in viscosity are among the most out- 

 standing characteristics of those chemical substances which enter 

 into the composition of protoplasm. It is to be expected there- 

 fore that viscosity changes play a large part in the mechanics of 

 vital processes. 



In colloidal solutions viscosity changes may be of two sorts. 

 Oftentimes the change is only slight but in other cases there is so 

 marked an increase in viscosity that the colloidal solution loses 

 many of the properties of a liquid and is apparently a solid. Of 

 course it is not truly a solid, for the solid state is properly asso- 

 ciated with crystal form. Highly viscous colloidal liquids are 

 known as semi-solids or gels. There is no sharp boundary line 

 between such gels and ordinary liquid colloidal solutions or sols. 

 Every intergradation exists. Colloidal solutions when treated 

 with reagents sometimes undergo gel formation but in other cases 

 there is a precipitation. In the latter instance there is often a 

 transitory increase in the viscosity of the liquid, accompanied 

 perhaps by the formation of an unstable gel, then as the colloidal 

 substance passes out of suspension, the viscosity of the liquid 

 decreases again. In a test-tube there is apparently a sharp dif- 

 ference between gelation and precipitation, but this would not be 

 true within a cell. The entire cell is often smaller than a single 

 flake of the precipitate. 



From the first the importance of protoplasmic viscosity changes 

 has been clearly recognized by biologists. Many theories of 

 diverse vital processes have been based on the idea of a change 

 in viscosity or a change in state of aggregation. Claude Ber- 

 nard himself believed that anesthesia was due to a " semi-coagu- 

 lation " of the protoplasm. 1 Since Claude Bernard many have 

 held a similar view. 



In order to support this view, some authors have endeavored 

 to show a physical effect of anesthetics on various colloidal solu- 

 tions. Hober and Gordon 2 showed that chloroform vapor and 



1 Bernard, C., Lec.ons sur les anesthesiques et sur 1'asphyxie, Paris, 1875. 

 3 Hober, R., and Gordon, Dora, Beitr. Chem. Physiol. u. Path., 1904, V., 432. 



