4 o MOLECULAR WEIGHT AND POLYMERISM 



| B J H 2 



\\ 



I 1 



but which, however, are of the highest importance in 

 testing the laws of osmotic action. 



We may first mention a measurement of Ramsay on 

 gases l which is very instructive as to the mechanism by 

 means of which osmotic pressure is produced. A palladium 

 vessel A (Fig. 6) constituted the mem- 

 brane and contained nitrogen, whose 

 pressure could be measured, as shown 

 in the figure. A was then surrounded by 

 a stream of hydrogen at known pressure, 

 and the pressure inside A rose, on account 

 of the gas passing through the palladium 

 partition, by an amount nearly equal to 

 the external hydrogen pressure (would 

 perhaps have coincided with it if the 

 external atmosphere had not originally 

 contained more hydrogen). Therefore the (osmotic) excess 

 of pressure of the (dissolved) nitrogen present in the 

 hydrogen corresponded to the pressure of the nitrogen 

 alone (as Avogadro's law applied to solutions requires). 



Next we have Pfeffer's measurements of solutions. He 

 used a semipermeable membrane of copper ferrocyanide, 

 but gave it sufficient strength by depositing it inside a 

 small porous battery-cell, the cell (moistened) being filled 

 with potassium ferrocyanide, and dipped in copper sulphate. 

 Diffusion from both sides then brings about the formation 

 of the membrane. With such an apparatus the following 

 measurements were carried out on a one per cent, sugar 

 solution (i gm. in 100-6 c.c.). 



. 6. 



Temp, (t) 



7 

 T 4 



22 

 32 

 36 



Osm. press. (P) 



o 664 atm. 

 0.681 

 0-721 ,, 

 0.716 ,, 

 0.746 



M = 0-813 x 



343 

 343 

 333 

 346 

 337 



273 + t 



1 Zeitschr.f. Phys. Chem. 15. 518. 



2 Osmotische Untersuchungen. Leipzig, 1877. 



