2 
Proceedings of the Royal Society 
Monday, 1st December 1884, 
THOMAS STEVENSON, Esq., Memb. Inst. C.E., President, 
in the Chair. 
The President opened the Session with a short Address, and 
an Obituary Notice of Sir Alexander Grant. 
The following Communications were read : — 
1. On the Distribution of Energy between colliding Groups 
of Molecules. By Sir W. Thomson. 
2. On the Dynamics of Reflection and Refraction in the 
Wave Theory of Light. By the Same. 
3. On Kerr’s Discovery regarding the Reflection of Light 
from a Magnetic Pole. By the Same. 
4. On an Improved Method of Measuring Compressibility. 
By Professor Tait. 
When the compressibility of a liquid or gas is measured at very 
high pressures, the compression vessel has to be enclosed in a strong 
cylinder of metal, and thus it must be made, in some way, self- 
registering. I first used indices, prevented from slipping by means 
of hairs. Sir W. Thomson’s devices for sounding, at small depths, 
by the compression of air, in which he used various physical and 
chemical processes for recording purposes, led me to devise and 
employ a thin silver film which was washed off by a column of 
mercury. Much of my work connected with the Challenger 
Thermometers was done by the help of this process. Till quite 
recently I was unaware that it had been devised and employed by 
Cailletet in 1873, only that his films were of gold. 
