XIV. 
METHOD OF DETERMINING THE ABSOLUTE DILATATION 
OF MERCURY. 
By J. JOLY, 8c.D., F.R.S., 
Professor of Geology and Mineralogy in the University of Dublin ; 
Hon. Secretary, Royal Dublin Society. 
[Read, January 15; Received for Publication, January 18; 
Published, Frsruary 21, 1907.] 
Tne classical experiments of Dulong and Petit and of Regnault, 
whereby the absolute dilatation of mercury has been determined, 
necessitate the use of apparatus so complicated and costly as to 
render the experiments very difficult of repetition. A simple 
apparatus which wili enable the teacher or student to make the 
determination with accuracy and ease is worth describing. The 
apparatus which I now describe was made by me many years ago, 
but no formal account of it has appeared. 
The principle of the apparatus is as follows :—Ii a mercurial 
barometer be raised in temperature, it, of course, reads higher 
owing to the diminished density of the mercury. The amount of 
rise is independent of the expansion of the glass, and at once gives 
the absolute dilatation of the mercury, assuming that the pressure 
of the atmosphere has not varied during the progress of the experi- 
ment. In short, for the balancing column of mercury used in the 
experiments of Dulong and Petit, the steady pressure of the 
atmosphere is substituted. 
In order to gain some idea of the degree of accuracy to be 
expected, I take the following figures :— 
Let the initial temperature be 10° C, and the final temperature 
100°C. The tables give us 6 as 15°5707, and di as 13°3522. 
Jf the initial height is 76 cms., then as 4,6, = /262, we find that 
h, is 77°244, The rise has been 1°244 cms. . 
If we read this with surety to 0:005 cms., the degree of 
SCIENT, PROC. R.D.S., VOL. XI., NO. XIV. Z 
