486 
THE EARL OF BERKELEY AND MR. E. G. J. HARTLEY ON THE 
actuated by a screw and wheel, compresses the “ steam cylinder ” oil which lies 
between them. The plunger supporting the weights may be regarded as the pressure 
gauge of the instrument; it was kept slowly rotating hy hand whenever it was 
in use. The apparatus can be worked up to 136 atmospheres. The sensibility 
throughout the whole range is about 0'12 atmosphere, consequently at low pressures 
the percentage error is considerable. 
The apparatus was compared with a standard Bourdon gauge and found correct. 
It should he borne in mind that the total pressure on the solution is the pressure 
registered by the dead weight together with that of the atmosphere. 
The Semi-Permeable Membranes. 
The membranes were deposited on the surface of the porcelain tubes by the 
following means. The porcelain tube is placed in a copper sulphate solution 
(50 grammes in a litre) in a desiccator and the air exhausted, until no more bubbles 
come off from the tube—this takes place only after several days—the tube is 
withdrawn, wiped inside and outside with a clean linen duster, and allow-ed to dry 
for f hour. The ends are then closed by rubber plugs, perforated for the passage of 
glass rods ; then, holding the tube horizontal and spinning it rapidly between the 
fingers, it is plunged into a solution of potassium ferrocyanide (42 grammes in a litre). 
By this means an even deposit of copper ferrocyanide, very close to the outer surface 
of the porcelain, is obtained. The tube is allowed to soak in the ferrocyanide, after 
which it is set up for electrolysis. The same solutions and of the same strength are 
used ; the tube is plugged at one end, and at the other is fitted with a perforated 
plug and thistle funnel, through wdfich a copper electrode dips into the copper 
sulphate solution, while a platinum electrode is immersed in the ferrocyanide 
surrounding the porcelain tube. It was found best to place the platinum electrode in 
a porous pot suspended in the ferrocyanide solution, in order to prevent the alkali 
from attacking the membrane, and the solution in the pot was frequently changed 
during the experiment. 
The current is passed (from the copper to the platinum electrode) until the 
resistance becomes constant ; this generally takes about 2 hours. We found, as 
noted by Messrs. Morse and Horn*, that apparently the best voltage to apply is a 
hundred volts. When the resistance has risen to a steady value, the tube is taken 
out, washed and allowed to soak in distilled water for about ten days; care should be 
taken that the last traces of copper sulphate are not washed away. Experiment 
showed that unless this substance be present, the colloid copper ferrocyanide begins 
to dissolve. It may be mentioned that, before the necessity of this precaution w r as 
realised, we spoilt nearly the whole of our membranes; only one of them regained its 
efficiency and that only after several months’ wmrk on it. 
* ‘ American Chem. Jour.,’ vol. 26. 
