240 Wisconsin Academy of Sciences, Arts and Letters. 
rubber, thus completing the transference. On the other hand, 
those substances which are not soluble in rubber, i. e. are not 
taken up by the rubber, will obviously not pass through the 
latter. And so what is required is a solvent which will read¬ 
ily be taken up by rubber, without, however, disintegrating the 
same, and a solution of such a character that the solute shall 
not be soluble in rubber. The less affinity the solute has for 
rubber the better; for then when the solution is brought into 
contact with the rubber the latter will imbibe practically only 
solvent to the exclusion of solute, thus leaving the solution 
slightly more concentrated, that is to say making a partial sepa¬ 
ration of solvent and solute. 
In casting about for a suitable solvent very many substances 
were tried. It was of course desirable to secure a solvent that 
was not too volatile at ordinary temperatures or too obnoxious. 
In the course of this work it soon became evident that water, 
alcohols and in general compounds containing considerable hy¬ 
droxyl, relatively speaking, are not suitable for they are not 
taken up readily enough by rubber nor in sufficient quantity. 
On the other hand many compounds, like hydrocarbons, their 
halogen substitution products, carbon disulphide, ether, etc., 
though taken up readily by rubber and in considerable quantity 
gradually act upon the latter to such an extent as to form with 
it a very soft, sticky mass or even a liquid, a combination or 
solution then of such mechanical properties as to be entirely 
unsuitable for quantitative osmotic experiments. After trying 
a large number of liquids, pyridine was finally taken as being 
the most suitable for the purpose in hand. From its very na¬ 
ture pyridine is a substance which would perhaps be expected 
to fulfill the requirements. Its high carbon and hydrogen con¬ 
tent make pyridine almost a hydrocarbon, indeed, it dissolves 
in hydrocarbons in all proportions. Since rubber is a hydro¬ 
carbon, we should expect it to imbibe pyridine readily. It was 
found that pyridine is imbibed by rubber, increasing the bulk 
of the latter somewhat to be sure, but without otherwise mater¬ 
ially altering the mechanical properties of the rubber. Indeed, 
I found that vulcanized caoutchouc may even be boiled in pyri¬ 
dine for hours, in which case there is a slight amount of ma- 
