318 RESEARCHES ON EVAPORATION AND DISSOCIATION. 



The value of c, if the temperatures of mercury are chosen 

 as ordinates, is 0-0004788 ; if those of water are chosen, 

 0-0009792. It was more convenient to employ the constant 

 derived from the absolute temperatures of mercury. The 

 method of calculation was as follows : A diagram was con- 

 structed, on which the ordinates wore the absolute tem- 

 peratures of mercury, and the abscissaa the ratios of 

 absolute temperatures of mercury and water, at pressures 

 corresponding to the absolute temperatures of mercury. 

 From the equation ll'=B + c{t' — t), it is evident that the 

 points must lie in a straight line. A point was read, giving 

 the ratio at any one temperature ; the absolute temperature 

 of water was calculated from the ratio ; the vapour-pressure 

 of water corresponding to this temporaturo is the same as 

 that of mercury, inasmuch as the ratios refer to equal 

 pressures. Thus at an absolute temperature of mercury of 

 508°, tho ratio, as read from the line, was 1-6331. The 



absolute temperature of water was thoroforo ,-'...... =311-0()°. 



The vapour-pressure of water at 311-00°, ascertained from 

 Regnault's tables, is 49-466 mms. This is therefore tho 

 vapour-pressure of mercury at an absolute temperature of 

 508°. The ratios corresponding to other absolute tem- 

 peratures of mercury wore calculated from the equation 

 B = lt' + c{t'-i), the value of B being 1-6.331 when i( = 508°, 

 as given above. As the vapour-pressures of water are uncer- 

 tain below a pressure of 4'6 mms., it was necessary to calcu- 

 late the constants for a formula of the form log p = a + h a*'. 

 We append a table of comparison of results by different 

 experimenters ; the vapour-pressures for each degree at 

 higher temperatures have been already given on p. 314. 



