38 PHYSICAL CHEMISTRY IN BIOLOGY. 



loids do not gelatinize, do not swell up, and are readily precipitated 

 by electrolytes. To this group belong the metallic sols, the colloidal 

 metallic sulphides, and certain typical suspensions obtained by dissolving 

 water-insoluble substances in another liquid (alcohol, acetone) and then 

 pouring this solution into a large volume of water. In this way the 

 substance is precipitated in a finely divided condition. Such suspensions 

 behave in many respects like suspension colloids. Suspensions of mastic, 1 

 colophony, 2 and cholesterin 3 belong to this class. 



The hydrophile colloids stand closer to the crystalloids than to the 

 suspension colloids, and the transition between the crystalloids and the 

 hydrophile colloids is only gradual. At the boundary we find the pep- 

 tones and proteoses which belong to the proteins, but at the same time 

 dialyze rather well. On the other hand, we also have colloids which to a 

 certain extent form intermediary steps between the hydrophile colloids 

 and suspension colloids. These intermediary members are the colloidal 

 acids and metallic hydroxides, which correspond with the suspension col- 

 loids by being readily precipitated by electrolytes. In this case they 

 separate as gels which differ from the gels of the hydrophile colloids 

 by not being again soluble in water. Finally, there are also numerous 

 intermediary members between the suspension colloids and the finely 

 divided substances suspended in water (kaolin). 



Osmotic Pressure. As above stated, the osmotic pressure of solu- 

 tions of crystalloids can be determined only in exceptional cases by 

 means of the semipermeable membrane, because it is very difficult to 

 prepare membranes which are impermeable for crystalloids. As pre- 

 viously stated, most membranes are impermeable for colloids, and the 

 osmotic pressure of the colloids can be best directly determined by the 

 aid of a membrane in a so-called osmometer. As shown by MOORE 

 and ROAP, in such an apparatus changes in pressure can be determined 

 which are not detectable by the determination of the freezing-point. 4 



Equimolecular solutions of various non-electrolytes give the same 

 osmotic pressure. From this it follows that when different non-elec- 

 trolytes exist in solutions with the same percentage concentration, 

 the osmotic tension of these solutions must be in inverse proportion 

 to their molecular weights. Certain colloids which will be discussed in 

 another connection (proteins, glycogen, etc.) must have a very large 

 molecule. From this it follows that these bodies must exert a very 

 low osmotic pressure. The proteins always contain a small amount 

 of salts which exist either in a sort of combination with the colloids 

 or are to be considered as contaminations which are 'difficultlv removed. 



1 Zeitschr. f. physik. Chem., 57, 47, 1906. 3 Bioch. Zeitschr., 7, 152, 1908. 



8 Ibid., 38, 385, 1901. 4 Bioch. Journ., 2, 34, 1906. 



