Methods for Determining Osmotic Pressures. 169 
is placed in a Petri dish containing water; this makes measurement 
easier and keeps the temperature constant. Fig. 1 shows the 
arrangement of the tubes, the black drops represent the salt 
solution of known strength, those shaded, the cell sap. 
In this way in each tube there are a number of small chambers 
full of air, the end walls of each chamber being made by the surfaces 
of the two solutions. Evaporation of the solvent takes place from 
both surfaces into the closed air space, but condensation takes 
place more rapidly on the surface of the stronger solution. The 
drops of the stronger solution therefore increase in length while 
those of the weaker decrease. 
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Fig. 1. For description see text. 
As soon as the tubes are ready the lengths of the drops are 
measured. The first and last drops are not measured as their con* 
centration may have been slightly altered when the ends of the tube 
were sealed. The drops are measured, under a low objective 
(§ inch) of the microscope, by means of a micrometer eye-piece. 
The middles of the menisci of each drop are focussed clearly and 
the distance between them read off on the scale. The appearance 
of the drop and the scale is shown in Fig. 2. 
Fig. 2. For description see text. 
The scale I used had 100 divisions. With care readings may be 
made to a tenth of one of these. Errors in reading were found not 
to exceed a fifth of one of the divisions. 
