﻿Volume, and Temperature of Rarefied Gases. 305 



measuring-vessel ; it communicated with two lateral descend- 

 ing tubes, through one of which gas could be introduced 

 at will. It was closed by raising a reservoir of mercury, 

 and covering an opening more than thirty inches below the 

 top of the measuring-vessel ; but when the reservoir was 

 lowered, gas could be passed into the measuring-vessel, 

 through the capillary tube. The second descending tube 

 connected the measuring- vessel with a wide manometer 

 formed like a siphon ; and by means of a reservoir it was 

 possible to raise the mercury in the open limb to a mark 

 with great accuracy. Corrections were introduced for tem- 

 perature, measured by a differential air-thermometer, and 

 also for the varying volume of the apparatus due to the 

 different heights of the mercury in the manometer. It is 

 easy to see how this apparatus could be employed to test 

 Boyle's law. To test Gay-Lussac's, he describes the operation 

 thus : — 



" The mercury is run out of the measuring-vessel to a 

 mark on the capillary tube, and weighed. Let this be done 

 at the temperature of boiling water. Now cool the bath 

 to 0° by surrounding the vessel with ice — the pressure begins 

 to diminish as is indicated by the manometer ; but we will 

 not allow it to do so. By means of the stopcock and the 

 other capillary, we let mercury run into the vessel until the 

 pressure of the gas at 0° becomes the same as it was at 100°. 

 The apparatus is then raised to the ordinary temperature, and 

 the mercury which entered the tube at 0° is run out and 

 weighed. This weight and the previous one give us the 

 data necessary for calculating the true coefficient of expansion 

 of gases." 



No final numbers are given in MendeleefTs published 

 papers, so far as we have been able to discover ; but the 

 general result is stated thus : — 



The product of pressure into volume, pv, which according 

 to Boyle's law should be constant, varies considerably with 

 diminution of pressure; at low pressures pv/dp is positive,?', e. 

 the gas is less compressible than it would be if it followed 

 Boyle's law. Stating the case broadly, it may be said that at 

 low pressures gases approach the state of a liquid or a solid ; 

 or pv increases with decrease ofp; or e/d increases, for d= 1/v. 



We have thought it advisable to give a tolerably full 

 description of Siljestrom's and MendeleefTs experiments, 

 because they are described in languages not generally known; 

 we may discuss the remaining researches on the subject in 

 much shorter space, seeing that they are described in French 

 and in German. 



