204 
Proceedings of the Royal /Society 
these latter cases a part of the current produced by the external 
electro-motive force must he considered as caused by electrolytic con- 
vection, and this part vanishes very slowly and never completely. 
The quantity of electricity which was carried to the platinum 
electrodes when they were free of hydrogen, was exactly proportional 
to the electro-motive force applied to the cell, down to forces of 
T oVu Daniell. From this is to be inferred that no kind of chemical or 
molecular force exists in the interior of the electrolytic fluid, which 
is able to prevent the ions of the electrolyte from moving about in 
the interior of the fluid, till they have got the distribution required 
for the equilibrium of the electric forces. For if such a force existed, 
we ought to find an inferior limit to the electro-motive forces, which 
are able to charge the electrodes like a condenser. 
2. On the Average Pressure due to Impulse of Vortex Eings 
on a Solid. By Sir William Thomson. 
3. On the Crushing of Glass by Pressure. 
By Professor Tait. 
In the course of my examination of the “ Challenger” thermo- 
meters, I was once surprised by a sudden bell-like vibration of the 
massive iron cylinder (nine inches thick) in which the apparatus to 
be tested was enclosed. This was due to the giving way of the 
protecting-sheath of one of the thermometers, at a pressure of a 
little over four tons weight on the square inch. When the pressure 
was let off, the bulb of the thermometer was found to have been 
crushed to an almost impalpable powder. I then determined to 
make a set of experiments with the view of finding under what 
amount of distortion glass gives way. For this purpose I employed 
the smaller compression apparatus which I have already described to 
the Society. Its capacity is about ten cubic inches only, so that a 
couple of strokes of the pump produce a pressure of three to four 
tons weight. 
I procured a number of pieces of glass tubing, with very varied 
ratios of external and internal diameter. These were all drawn 
from the same melting in the Leith Glass-works, and consisted of 
ordinary lead-glass. 
