566 



SCIENCE PROGRESS 



curved metal tubes VV of larger diameter are clamped against 

 the ends of the brass case EF, the joint being made tight by a 

 rubber washer. The case EF is filled and the pressure trans- 

 mitted through the aperture at R, whilst the aperture at S 

 serves to empty the vessel. 



The Pressure Apparatus. — The pressures were measured by 

 means of a dead-weight standard pressure-gauge, fig. 7. This 

 has two plungers, one supporting a series of weights, whilst 

 the other, actuated by a screw and wheel, compresses the 

 "steam cylinder" oil which lies between them. The plunger 

 supporting the weights, which forms the pressure-gauge of the 

 instrument, was kept slowly rotating by hand whenever it was 

 in use. The apparatus can be worked up to 136 atmospheres 



[= — | r - p—y 



Fig. 7. — Pressure apparatus. 



but as the sensibility is about o - i2 atmosphere throughout, the 

 percentage error is increased greatly at low pressures. 



The Semi-permeable Membranes. — To deposit the membranes 

 the porcelain tube was placed in a solution of copper sulphate 

 (50 grammes in a litre) in a desiccator and the air exhausted 

 during several days until no more bubbles appeared from the 

 tube ; the tube was then removed, wiped inside and out and 

 allowed to dry during three-quarters of an hour. After closing 

 the ends of the tube with rubber plugs carrying glass rods, it 

 was plunged with a spinning motion into a solution of potassium 

 ferrocyanide (42 grammes in a litre), allowed to soak and then 

 set up for electrolysis, the current being passed from a copper 

 electrode immersed in copper sulphate solution inside the tube 

 to a platinum electrode immersed in a ferrocyanide solution 

 outside the tube ; the platinum electrode was enclosed in a 

 porous pot to prevent the alkali liberated there from attacking 



