of Superheated Vapours, 403 



the vapour of the liquid or of air which they might still retain. 

 In this way the subsequent observations were certainly made 

 upon constant quantities of vapour and of air (so far as the ad- 

 hesion-phenomena to be afterwards considered did not come 

 into play), of which I in many cases convinced myself by re- 

 peated series of observations with falling and with rising tem- 

 perature. 



I now made the observations for a series of temperatures, 

 each of which was kept constant for a longer time by brisk 

 action of the stirrer ; and to read off the difference of level of 

 the mercury for each temperature a delicate cathetometer was 

 employed. The cathetometer (an excellent specimen made by 

 M. Schubart, of Ghent) stood upon a massive stone pillar which 

 is erected directly above the subterranean vault of our labora- 

 tory; the slightest shaking of the cathetometer is thus excluded. 

 The temperature of the bath was taken by a delicate Geissler's 

 normal thermometer which gives tenths of a degree. The sub- 

 sequent calculation shows, moreover, that a reading of the tem- 

 perature to tenths of a degree would really have been unneces^ 

 sary — that, in such observations of differences, it is much more 

 essential to have the same temperature in all parts of the tube; 

 and this, as I have said, was always attained by the perfect 

 effectiveness of the stirring-apparatus. 



It was then only necessary, besides the observations of the 

 level of the mercury, to note down at the same time with each 

 observation its position about the strokes which were marked on 

 the tube. 



After carrying out these observations, I broke off the eapiU 

 lary end of the tube on the vapour side, and thus put this por- 

 tion of the tube in communication with the external air. The 

 pressure p of the dry air in the other leg of the tube, at a fixed 

 temperature 6 of the bath, was then again determined, and the 

 position of the mercury about the diamond-marks noted down. 



Lastly the following weighings of the mercury were made : — 



(1) The weight of the mercury contained in the tube during 

 the experiments ; let it be W. 



(2) The weight of the mercury which, at a certain tempe- 

 rature 6 fills the whole tube; let it be x, 



(3) The weight of the mercury which at the temperature 6 l 

 fills the space occupied by the vapour during the mea- 

 surement when the position of the mercury about the 



diamond-marks was noted down ; let it b 



ey. 



(4) The weight of the mercury which at the temperature 2 

 fills the space occupied by the dry air at the time of the 

 determination of its pressure in comparison with that of 

 the external air ; let it be z. 

 2D2 



