﻿302 Mr. Baly and Dr. Ramsay on the Relations of Pressure, 



to pressure at constant temperatures ; and as there is an 

 intimate connexion between the compressibility of gases and 

 their thermal expansion, we think it desirable to give a short 

 summary of the work which has already been done on the 

 subject. It is obvious that if the product of pressure and 

 volume of a gas diminishes with the degree of rarefaction, 

 it is to be expected that its coefficient of expansion with rise 

 of temperature will increase ; whereas, if the pv increases as 

 observed by Mendeleeff and others, its coefficient of expansion 

 will probably diminish. 



HlSTOKICAL KESUME. 



1. Relation of Volume to Pressure at Constant Temperature. 



For more than twenty years the question of the validity of 

 Boyle's Law, applied to rarefied gases, has been the subject of 

 frequent articles in scientific journals. In 1873 Siljestrom 

 (Bihang till K. Svenska Vet. Akad. Handlingar, ii. p. 1) 

 published in Swedish an account of experiments to test this 

 important law. His method consisted in exhausting one of 

 two stout cylindrical metal vessels connected together by 

 means of a tube provided with an air-tight stopcock. Both 

 vessels were placed in a large reservoir of melting ice in order 

 to maintain temperature constant at 0°. Each vessel was 

 provided with a mercury manometer : and while the larger 

 of the two had no opening except the tube already men- 

 tioned, which connected it with the smaller vessel, the latter 

 was joiued by means of a brass tube with a stopcock to an 

 ordinary exhausting air-pump, also furnished with a mercury 

 gauge. 



The plan of operation was as follows : — The pressure of 

 gas in the smaller vessel was reduced by means of the 

 air-pump, communication with the larger vessel being cut off 

 by means of the stopcock, and the pressure of the gas in both 

 vessels was read on the mercury gauges attached to them, and 

 also on that attached to the air-pump. The pressure registered 

 by the gauge of the smaller vessel and by the air-pump gauge 

 should of course be the same. Communication with the air- 

 pump was then shut off. The stopcock between the two 

 vessels was then opened, and gas passed from the larger to 

 the smaller. After time had been allowed for equilibrium of 

 temperature and pressure to be re-established, the pressures 

 on the gauges were again read. The capacities of the vessels 

 being accurately known, the product of pressure and volume 

 could be calculated. 



Sixteen series of experiments, each consisting of fourteen 



