APPENDIX A. 



ON AN IMPROVED METHOD OF MEASURING COMPRESSIBILITY. 



" 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 com- 

 pression 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. 



" But the use of all these methods is very laborious, for the whole apparatus has 

 to be opened for each individual reading. Hence it struck me that, instead of 

 measuring the compression produced by a given pressure, we should try to measure the 

 pressure required to produce a given compression. I saw that this could be at once 

 effected by the simplest electric methods ; provided that glass, into which a fine 

 ■platinum wire is fused, ivere capable of resisting very high pressures without cracking 

 or leaking at the junctions. This, on trial, was found to be the case. 



" AVe have, therefore, only to fuse a number of platinum wires, at intervals, into 

 the compression tube, and very carefully calibrate it with a column of mercury which is 

 brought into contact with each of the wires successively. Then if thin wires, each 

 resisting say about an ohm, be interposed between the pairs of successive platinum 

 wires, we have a series whose resistance is diminished by one ohm each time the 

 mercury, forced in by the pump, comes in contact with another of the wires. Connect 

 the mercury with one pole of a cell, the highest of the platinum wires with the 

 other, leading the wires out between two stout leather washers ; interpose a galvano- 

 meter in the circuit, and the arrangement is complete. The observer himself works 

 the pump, keeping an eye on the pressure gauge, and on the spot of light reflected by 

 the mirror of the galvanometer. The moment he sees a change < if deflection he reads 



1 Proc. Roy. Soc. Edin., vol. xiii. pp. 2, 3, 1884. 



