Kahlenberg—Osmosis and Osmotic Pressure. 247 
terial extracted from the rubber, giving the liquid a brownish 
color; but the rubber is not disintegrated or affected materially 
otherwise. The rubber used as membranes in the actual quanti¬ 
tative measurements was in fact thus extracted with boiling 
hot pyridine. After such treatment rubber when dried has 
practically all of its original properties; it is perhaps a little 
easier to rupture it by stretching it hard. It might at first be 
somewhat surprising that pyridine does not disintegrate rubber 
more. However, while pyridine is closely akin to hydrocarbons 
and consolute with them, it must be borne in mind that water, 
which has but little affinity for hydrocarbons, also is consolute 
with pyridine. Pyridine is then a rather unique substance, and 
it is hardly surprising that it should be imbibed by rubber 
sufficiently for the purpose in hand without unduly disintegrat¬ 
ing it. Direct experiment showed that at room temperature 
(about 20° C.) 100 grams of the vulcanized caoutchouc used 
imbibed 144.42 grams of pyridine in 24 hours, while the amount 
imbibed in 17 days was 145.17 grams. 
A suitable solute would be one that is soluble in pyridine yet 
insoluble in hydrocarbons. For instance, a substance soluble in 
petroleum or benzene would in general also be soluble in pyri¬ 
dine, but it would also be soluble in rubber (a hydrocarbon) 
and hence would pass through the latter. Thus oleic acid, the 
oleates of the heavy metals, ferric chloride, naphthalene, cam¬ 
phor are soluble in hydrocarbons, are consequently readily 
taken up by rubber, and when dissolved in pyridine pass 
through the rubber when the latter in an osmotic experiment 
separates the solution from pure pyridine. Because of the pe¬ 
culiar nature of pyridine already alluded to above, this liquid 
dissolves a goodly number of substances which are insoluble in 
hydrocarbons. The solubility of such substances in pyridine is 
to be sure rather limited as to quantity as a rule. So cane 
sugar, silver nitrate, lithium chloride are insoluble in hydrocar¬ 
bons and yet reasonably soluble in pyridine. One would con¬ 
sequently expect that when solutions of either of these sub¬ 
stances in pyridine be separated from pure pyridine by means 
of a rubber septum, practically none of the solute would pass 
through the rubber; in other words, the latter would be prac- 
