A. C. Halket. 
17 2 
regularly, the drops do not increase and decrease by the same 
amount. The change is not quantitative, the result simply shows 
that the drops of the stronger solution increase in length while those 
of the weaker decrease. Experience has shown that in order to get 
this regularity of increase and decrease great care must be taken in 
filling the tubes. The drops must not be allowed to run up and 
down the tube during filling, for if they do so the solutions become 
mixed as there is always a film of liquid on the sides of the tubes. 
This mixing does not so much affect the change when the difference 
between the concentrations of the solutions is greater, but with 
small differences it prevents a satisfactory result being obtained. 
The time it is necessary to leave between the first and second 
readings also varies with the strength of the solutions used. If the 
solutions differ very much from each other in strength, two to three 
hours is all that is necessary, but if the solutions are nearly equal 
in strength, e.g., *12 and *11 molecular solution, the tubes must be 
left for a longer time. It was generally found convenient to fill the 
tubes one day, measure the drops, then leave them all night and 
re-measure the drops the next morning. 
Values of the Osmotic Pressures of Cell Sap Found 
by Barger’s Method. 
In the tables below are given certain values of the osmotic 
pressure of the cell-sap of some succulent plants obtained by this 
method. 
Salicornia ramosissima (from water channel). 
Gram-Molecular Concentration of NaCl. 
1-0 
1-1 
1-2 
1-3 
1st 
2nd 
1st 
2nd 
1st 
2nd 
I St 
2nd 
Read- 
Read- 
Cha nge. 
Read- 
Read- 
Change 
Read- 
Read- 
Change. 
Read- 
Read- 
Change. 
ing. 
mg. 
<ng. 
ing. 
mg. 
ing. 
mg. 
ing. 
38-0 
42-9 
+ 
15-8 
19-1 
+ 
35-1 
34-0 
18-0 
15-6 
75-0 
69-8 
— 
400 
38-9 
— 
38-0 
39-0 
+ 
35-8 
3t;*8 
+ 
27-8 
34-7 
+ 
26-1 
28-3 
+ 
21-9 
20-0 
' 
18-9 
16-4 
The vapour pressure of the cell sap is less than that of a 1-1 
gram-molecular solution of sodium chloride and greater than that 
of a P2 solution, therefore the osmotic pressure of the cell-sap is 
taken to be equivalent to that of a P15 gram-molecular solution of 
this salt, i.e., to 45-6 atmospheres at 18"C. 
