of dark Heat-rays by Gases and Vapours. 7 



will become filled with vapour. If the tap leading to the fluid 

 be closed and the other opened, communication is made between 

 M and the large tube, and the vapour contained in M expands 

 throughout the whole tube. This process may be repeated as 

 often as desired ; and so the tube may be gradually filled with 

 continually increasing quantities of vapour. The ratio in 

 which the vapour contained in M expands when it comes into 

 the large tube may be obtained from the cubic contents of the 

 latter, which Tyndall gives incidentally as 220 cubic inches. 

 For the sake of clearness, we will take an example (that of 

 ether-vapour) in Tyndall's own words. Let us assume that 

 M contained jqq- - cubic inch: — " The vapours, on entering 

 the tube, have only the tension corresponding to the tempera- 

 ture of the laboratory, viz. 12 inches. This must be multiplied 

 by 2*5 iu order to give the atmospheric pressure. If, then, 

 the joVo cu bic inch, whose absorption, as shown, can be mea- 

 sured, expands into a space of 220 cubic inches, it would have 

 a tension of 



111 1 



x 7^ x 



220 2-5 1000 500000 

 of an atmosphere ! " 



To examine the accuracy of this method, we will choose as 

 examples sulphide of carbon and benzol, because the tables 

 admit of a control for these substances. 



From the table for sulphide of carbon we take: — 



(Table VI.) Sulphide of Carbon. Unit volume = \ cubic inch. 

 Volumes . 1-0 2-0 . . . 7*0 8-0 9-0 13-0 14-0 15*0 

 Absorptions 2*2 4'9 . . . 13*8 14-5 15-0 17-5 18*2 1 ( J'0 



Moreover, Tyndall gives for the absorption at a mercury- 

 pressure of \ and 1 inch the numbers 14*8 and 18*8. 



Now there can be no doubt of the conclusion that, whenever 

 equal absorptions take place, there must be present equal 

 numbers of molecules of vapour, and hence at the same tem- 

 perature there must be equal vapour-pressures. At \ inch 

 pressure the absorption, was 14*8; and the same absorption 

 takes place, according to the table, when M has been emptied 

 into the large tube about 8*6 times; and there must then have 

 been in the tube a pressure of \ inch. From this we can 

 easily calculate what the pressure in M must have been each 

 time for this result to have been obtained. 



If x denote the pressure in M, then when M is put into 

 communication with the large tube there will be a pressure of 



x 

 ^rr in the tube, since the vapour expands into a space 220 



times its own volume, supposing M to have a volume of 



