EXPERIMENTAL WORK ON OSMOTIC PRESSURE 631 



made with pyridine solutions and a semi-permeable membrane 

 of vulcanised caoutchouc, finds that the gas laws do not hold. 

 The osmotic pressures observed by him are far short of the 

 values required by the gas laws, and the changes of pressure 

 that accompany changes of temperature are very much greater 

 than they would be if osmotic pressure were proportional to 

 the absolute temperature. If the experimental work brought 

 forward by Prof. Kahlenberg were adequate and sound, these 

 results would be of the utmost importance, but some of the 

 experiments are open to serious criticism, and the work is 

 not weighty enough to overthrow the results reached by other 

 workers, and already described in this paper. Future investiga- 

 tion will probably bring some explanation of the conflicting 

 evidence. 



The Osmotic Pressure of Colloidal Solutions 



Among the investigators who have contributed to the 

 comprehension of osmotic phenomena are to be found many 

 physiologists and botanists. Pfeffer himself, whose classical 

 researches form the foundation of our knowledge of the subject, 

 was Professor of Botany at Basle, and many others besides him 

 have attacked the problem from the biological side. Quite 

 recently another interesting contribution to our knowledge has 

 been made from a similar quarter in the form of an investigation 

 of the osmotic pressure of colloidal solutions by Prof. Benjamin 

 Moore and Dr. Roaf. 1 



Colloidal solutions have been the subject of extensive study 

 in recent years, and the current view of a colloidal solution as 

 a suspension of very finely divided particles has led frequently 

 to the impression that it would possess no osmotic properties 

 such as are shown by solutions of true crystalloids. While it 

 is undoubtedly true that in some cases the colloid "solution 

 aggregate" is so large that measurable readings of osmotic 

 pressure cannot be obtained, the work just referred to has 

 shown that many colloids in solution give a distinct and definite 

 osmotic pressure. 



The membrane used by Prof. Moore and Dr. Roaf consisted 

 of parchment paper, which, although it allows free passage to 

 water and such dissolved substances as uric acid and even 



1 Biochemical Journal, 1906, 2, 34. 



