On the Elastic Force of the Vapor of Mercury. 293 



ed as contradicting this result ; for although the delicate methods 

 employed by that chemist, enabled him to detect vapor at tempera- 

 tures as low as 32°, it may readily be conceded, that below this the 

 exceeding tenuity of the vapor might have caused it to escape his re- 

 searches. But even if the limit determined by Faraday, be consid- 

 ered absolute, it may be attributed to a physical cause, entirely inde- 

 pendent of the tension of the vapor, and by which the formation of 

 vapor is suddenly checked. 



The formula gives for the tension of mercurial vapor at 21.2° F. 

 6=0.00003889 atm. = 0.001 164 inches, or less than twelve ten thou- 

 sandths of an inch ; an elasticity which may be considered as insen- 

 sible in experiments on the elasticity of its vapor. 



The formula, therefore, although empirical, not only represents the 

 results of experiments between 446° F. and the boiling point of mer- 

 cury, 680° F., but also the observations on its vapor at temperatures 

 as low as the freezing point of water. It maybe used, therefore, to cal- 

 culate a table of tensions from the boiling point of water, up to that of 

 mercuiy, the temperature being estimated by the mercurial thermom- 

 eter. A table thus calculated will be found at the end of this extract, 

 and may be considered as a condensed expression of the results of 

 my experiments. It is probable that the estimation of these tensions 

 may lead to the application of a correction in certain experiments, in 

 which no correction has been used, owing to uncertainty as to its 

 amount. This table can hardly be considered accurate beyond hun- 

 dredths of an inch, although I have carried out the jSgures given by 

 the formula. 



In the table the temperatures are given in intervals of degrees, 

 while in the formula, a range of a 100 degrees v^^as taken as unity. 

 The formula expressed for degrees of Fah. would be,* log. e 

 =—0.0035909^+0.0000023443^- - 0.000000031 64^^ 



The degrees are reckoned from the boiling point of mercury, 

 (680° F. ;) that is, the value of t will be found by subtracting the 

 given temperature from 680° F. One column of the table gives 

 the tensions in atmospheres, and a second in inches of mercury. 

 The numbers of the second column would be found from the formu- 

 la, by adding to log. e, which it gives, the logarithm of 29.94. I 

 may remark that from the elasticity of mercurial vapor, at different 



* In the Centigrade scale, log. e= — 0.0064637 <+0.0000075956 <2. 

 0.00000018452 f3. 



Vol. XXIV.— No. 2. 38 



