A . C. Halket. 
168 
Errera to be due to the fact that, as the vapour pressure of the salt 
solution was lower than that of water, the water vapour in the air 
of the damp chamber condensed more quickly on the surface of the 
hanging drop, the stronger solution. The drop therefore increased 
in size at the expense of the water in the cardboard. Barger 
investigated the phenomenon quantitatively and found that the best 
way to estimate the amount of change that took place in the volumes 
of any two solutions was to put drops of the solutions in a capillary 
tube and measure their increase or decrease under a microscope. 
The vapour pressure of the cell sap is compared with that of 
each member of a series of salt solutions of which the concentration 
is known. These salt solutions differ only slightly in strength one 
from another, so that there is one whose vapour pressure will be 
approximately equal to that of the cell sap, the solution under 
investigation. When such a solution is found it is regarded as iso¬ 
tonic with the cell sap. The difference in vapour pressure of the 
two solutions, the cell sap and one of the solutions of known 
strength, is found by measuring the increase or decrease in the size 
of the drops of the solutions in a capillary tube. 
Capillary tubes about three inches in length are used, and are 
filled as follows. One end of the capillary tube is closed by one’s 
finger and the other end is dipped into the solution of known 
strength and a drop is allowed to run up the tube. The tube is then 
removed from the solution and a bubble of air is made to enter it, 
and it is then placed in the cell sap and a drop of that is allowed to 
run up, then another bubble of air, then another drop of the solution 
of known strength and so on. The process is repeated until the 
tube contains a number of drops of the two solutions alternating 
with each other, each drop being separated from the one next to it 
by a bubble of air. The size of the drops is regulated by pressure 
of the finger on the top of the tube. The first and last drops, which 
should be of the solution of known strength, are made much larger 
than the others. The number of drops is immaterial and is a matter 
of convenience. When there are a sufficient number of drops in 
the tube, the ends are sealed up and the tube, for convenience in 
handling and identification, is fastened to a microscope slide either 
with Canada balsam or with soft sealing wax. Other tubes are 
filled with one of the different members of the series of salt solutions 
of known strength and the cell sap. The slide, when full of tubes, 
