Farce of Aqueous Vapour at Atmospheric Temperatures. 391 



absolutely dry, the stop-cock was closed, and the small tube connect- 

 ing the air vessel with the pump having been drawn out in the mid- 

 dle, and sealed hermetically by means of a spirit lamp, the air ap- 

 paratus was separated from the potash tube, and transferred to a tall 

 jar containing mercury, after which the sealed end of the small glass 

 tube was broken beneath the surface of the quicksilver. The ap- 

 paratus, however, being now completely filled, it became necessary 

 to remove some of the air, and this was done by opening the stop- 

 cock very gradually, care being taken that during this manipulation 

 the external mercury should be higher than its level within the tu- 

 bular portion. The entire was then placed in a small room, the 

 temperature of which was found not to vary more than one degree 

 Fahrenheit during the twenty-four hours, the stop-cock having been 

 first attached to one extremity of a string, which was carried over a 

 fixed pulley placed in the ceiling, and whose other end carried a 

 counterpoise by which the air vessel was kept in a vertical position, 

 and the observer was enabled readily to bring the mercury within 

 and without to the same level, before he registered the volume of 

 the included air. 



On the next day, after the apparatus was mounted, and the four 

 following ones, the volume of the dry air, its temperature, and the 

 existing pressure were accurately noted. This pressure, which was 

 measured by a portable barometer of Newman's, having undergone 

 a variety of corrections, for the capacity of the cistern compared to 

 that of the tube, for the excess of the temperature of the quicksilver 

 over 32°, for capillarity, and for a constant error by which I found 

 my barometer affected, when compared with the standard instru- 

 ment in the Observatory of Trinity College, I reduced by calculation 

 in each instance the observed volume of air to what it would be at 

 32°, and under a pressure of 30, using for the expansion of air the 

 corrected coefficient ^^, which has resulted from the experiments 

 of Rudberg, and thus obtained the following numbers, which, it 

 will be observed, differ very little from each other : — 



1 911-11 



2 911-85 



3 910-21 



4 913-30 



5 911-72 



911*64, therefore, the mean of the five observations, may be as- 

 sumed as the true volume of the included dry air, at 32°, and under 

 a pressure of 30. 



The volume of the dry air being determined, the next step was to 

 charge it with moisture. In order to accomplish this, the air vessel 

 was lifted by means of the string, so as that the mercury within should 

 be about an inch higher than the external mercury, and distilled 

 water was then poured into the upper cavity of the stop- cock, so as 

 completely to fill it. The stop-cock was now cautiously turned, so 

 as to admit the entrance of the moisture guttatim • and more water 

 being occasionally poured on, this manipulation was repeated until the 

 mercury within came to be covered by a film of water of about one- 



