248 Wisconsin Academy of Sciences , Arts and Letters. 
tieally impermeable for sugar, silver nitrate and lithium chlor¬ 
ide. Now this is actually what was found in the qualitative 
tests described above. These substances pass through rubber 
in extremely slight quantities which are quite comparable with 
the amounts of cane sugar that pass through the much studied 
tests described above r These substances pass through rubber 
an aqueous sugar solution. However, on account of the fact 
that sugar has a high carbon and hydrogen content, one would 
expect it to have more affinity for a hydrocarbon than either 
silver nitrate or lithium chloride, and that consequently it 
would pass through rubber a little more readily than these salts. 
Experiment has also shown that this is actually the case; 
though as stated above, the amount of sugar which passes 
through the rubber membrane is quite small. 
The quantitative measurements of osmotic pressures were 
then made by using solutions of cane sugar, lithium chloride, 
and silver nitrate in pyridine, these solutions being separated 
in each case from pure pyridine by means of a membrane of 
vulcanized caoutchouc previously treated with boiling hot pyri¬ 
dine so as to extract any soluble ingredients. It was not the 
purpose of the quantitative measurements of osmotic pressures 
to produce and measure enormous pressures; though as was 
shown above in No. 22 a pressure of approximately fifteen at¬ 
mospheres was actually measured in the case of a normal solu¬ 
tion of silver nitrate in pyridine. The efforts were rather di¬ 
rected toward determining with a sufficient degree of accuracy 
moderate pressures, using different concentrations of the solu¬ 
tions employed at several different temperatures. 
The osmotic cells were made entirely of glass, excepting of 
course the surface actually closed by the membrane itself. The 
different parts of the cells were fused together so as to form 
one piece, thus avoiding cemented joints of any kind. Figure 
6 shows how these cells were made. To a stout, carefully made 
thistle tube having a flare of about 45 degrees at E, a T was 
attached, the tube being provided with a bulb and bent as shown 
in the figure, C. To C was fused a manometer tube D having 
a bore of about 0.5 mm.; this tube was made as long as the ex¬ 
periment required. The small bulb and bent part of the tube 
