30 PHYSICAL CHEMISTRY IN MEDICINE. 



from the solid to the liquid, depending only upon the 

 amount of water they have absorbed. Chemical reactions 

 may take place anywhere in such a medium, and with 

 almost the same velocity as in the fluid absorbed by the 

 colloid alone. Such a jelly does not take up any other 

 colloid which is brought in contact with it (differentiation 

 between different membraneless cells), and a foreign 

 colloid imbedded in it does not tend to spread (intra- 

 cellular differentiation). Such gels undergo most deli- 

 cately shaded changes in state even without changes in 

 temperature, through the action of substances which are 

 present in the living organism. They may be rendered 

 more solid or more fluid, without suffering a change in 

 the amount of water which they hold, through the action 

 of crystalloids, and also through the action of certain 

 enzymes (partial or complete peptonization). 



Such solid colloids show yet other properties that 

 have been used as potent arguments in favor of the 

 entirely fluid character of living matter. If mercury 

 globules are driven under pressure through a capillary 

 into solidified gelatine, the gelatine closes in as completely 

 behind the rapidly moving globules as a liquid itself. 

 The separation of droplets such as ALBRECHT has de- 

 scribed in protoplasm is also possible in solidified gelatine. 

 In the fluid state, that is to say, above the solidification- 

 point, gelatine is precipitated by certain electrolytes such 

 as the sulphates, citrates, and tartrates of the alkali 

 metals. As can be proved microscopically and mechan- 

 ically, this precipitation is a separation of droplets 

 the appearance of a "phase" richer in gelatine. The 

 precipitating power of the electrolytes decreases with an 

 increase in the temperature, that is to say, a more con- 



