232 Wisconsin Academy of Sciences, Arts and Letters. 
passage of pyridine through the membrane to the solution. 
Thus the reason why caoutchouc is a “semipermeable” mem¬ 
brane in these cases is given; and we should expect in all 
cases in which the solute employed is insoluble in hydrocarbons, 
like kerosene, benzene, etc., yet is soluble in pyridine, that vul¬ 
canized rubber will act as a “semipermeable” membrane when 
it is employed in separating pyridine from the pyridine solu¬ 
tions of such solutes. This has been confirmed in the case of 
cane sugar and lithium chloride which are soluble in pyridine 
yet insoluble in hydrocarbons. The experiments are given be¬ 
low. Conversely, when a substance is soluble in hydrocarbons 
as well as in pyridine, that substance will always pass through 
vulcanized caoutchouc in notable quantities when its solution 
in pyridine is separated from pure pyridine by means of the 
caoutchouc septum. Examples of such cases will also be found 
below. Though experiments 19 to 22 are only quasi quantita¬ 
tive in character they are already quite sufficient to show that 
here the osmotic pressure does not follow the gas laws at all. 
The change of the pressure with temperature is very much 
greater than proportional to the absolute temperature; and 
again the pressure varies much more rapidly with change of 
concentration of the solute than is required by Boyle’s law. 
Experiment 20 reveals the fact that at room temperature the 
osmotic pressure of the 0.05 normal solution of AgNG 3 in pyri¬ 
dine is practically nil under the conditions described, while on 
the basis of the vant’ Hoff theory the osmotic pressure of this 
solution ought to be over an atmosphere. We have in No. 20 
the case where the solution has been diluted to such a point 
that its affinity for additional pyridine is practically equal to 
the affinity between pyridine and the rubber, so that the latter 
can not be robbed of its pyridine content by the solution, and 
consequently the liquid in the osmotic cell does not increase in 
bulk. 
The observations made in Nos. 14 and 16 are such as might 
have been foreseen considering the fact that water has practi¬ 
cally no affinity for rubber; that the latter has considerable 
affinity for pyridine; that water and pyridine are consolute 
liquids; and that silver nitrate, though soluble in water and 
