16 GENERAL AND PHYSICO-CHEMICAL. 



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 do 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. 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 

 ROAF, 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 con- 

 sidered as contaminations which are difficult to remove. For this reason 

 it has been repeatedly stated that these salts are responsible for the 

 small differences in the osmotic pressure. By carefully washing crys- 

 stalline proteins from serum and egg-white, REID was able to prepare 

 bodies which gave finally no osmotic pressure in the osmometer. 5 In 

 opposition to this, MOORE and ROAF as well as LILLIE call attention to 

 the fact that the osmotic pressure of protein solutions is influenced by 

 the treatment which the protein received before the determination. 



1 Zeitschr. f. physik. Cbem., 57, 47 (1906). 



2 Ibid., 38, 385 (1901). 



3 Bioch. Zeitschr., 7, 152 (1908). 

 Bioch. Journ., 2. 34 (1906). 



6 Journ. of Physiol., 31, 438 (1904). 



