492 
THE EARL OF BERKELEY AND MR. E. G. J. HARTLEY ON THE 
rates, one above and one below the turning point, are then considered to be sufficient 
to enable that point to be calculated. 
With viscous solutions, such as 660 grammes of cane sugar, it was necessary to 
lengthen the interval of time considerably ; in fact, a constant rate was generally only 
attained after the apparatus had been set up for some 5 or 6 hours. 
In the earlier experiments, before we realised that the method was susceptible of 
greater accuracy, we used somewhat larger increments of pressure. It was found that 
there were slight differences in the rate corresponding to a given pressure according 
as that pressure had been reached from above or below. A moment’s consideration 
will show the cause of this. 
It is evident that for any definite pressure, unless it be exactly the turning-point 
pressure, the solution near the membrane must be changing in concentration; the 
change is brought about by the water which is either squeezed out or sucked into the 
solution. This layer of abnormal concentration is prevented from merging rapidly 
into the remainder of the solution, because it is entangled in the pores of the porcelain 
tube lying outside the membrane, so that on taking off weights, when working above 
the turning point, the observed rate is that due to a slightly more concentrated 
solution than normal, while when working below it is due to a more dilute one. 
The following table gives an example of the kind of effect produced. It is 
extracted from the laboratory notebook, and gives part of the experiment of 
August 23, with 420 grammes of cane sugar. 
Table II. 
Temperature of 
osmotic apparatus. 
Pressure on the 
solution in 
atmospheres. 
Gauge readings. 
Rate of movement 
in millimetres per 
15 minutes.* 
5.10 p.m. 
° C. 
0-13 
41-80 
1 
5.12 „ 
— 
41-80 
276-6 
- 12-2 
5.20 „ 
— 
42-77 
270-1 
5 22 
— 
42-77 
271-4 
- 5-1 
5.30 „ 
— 
43-74 
268-7 
5.32 „ 
— 
43-74 
270-8 
+ 0-9 
5.40 „ 
— 
44-71 
271-3 
5.42 „ 
— 
44-71 
274-4 
+ 7-1 
5.50 „ 
0-15 
45 ■ 68 
278-2 
5.52 „ 
— 
45 • 68 
282 • 2 
+ 13-9 
6.0 „ 
— 
44-71 
289-6 
6.2 „ 
— 
44-71 
288-1 
+ 6-4 
6.10 „ 
— 
43-74 
291-5 
6.12 , 
— 
43-74 
289-8 
+ 0-2 
6.20 „ 
— 
42-77 
289-9 
6 22 
— 
42-77 
287-3 
- 5-6 
6.30 ,, 
— 
41-80 
284-3 
6.32 „ 
— 
41-80 
281-0 
-11-6 
6.40 „ 
0-18 
42-77 
274-9 
6.42 „ 
— 
42-77 
276-2 
- 4-3 
6.50 „ 
— 
42-77 
273-9 
* The rate of movement is placed opposite the pressure causing it. 
