l6 PHILOSOPHICAL TRANSACTIONS. [aNNO 1785. 



meters, bent in the manner represented by abc, fig. 5, the bent end of which, 

 after being previously filled with quicksilver, was introduced, as in the figure, 

 under the glass def, inverted into water, and filled with the proper kind of air, 

 the end c of the tube being kept stopped by the finger ; then, on removing the 

 linger from c, the quicksilver in the tube descended in the leg bc, and its place 

 was supplied with air from the glass def. Having thus got the proper quantity 

 of air into the tube abc, it was held with the end c uppermost, and stopped with 

 the finger ; and the end a, made smaller for that purpose, being introduced into 

 one end of the bent tube m, fig. 4, the air, on removing the finger from c, was 

 forced into that tube by the pressure of the quicksilver in the leg bc. By these 

 means I was enabled to introduce the exact quantity I pleased of any kind of air 

 into the tube m ; and, by the same means, I could let up any quantity of soap- 

 lees, or any other liquor which I wanted to be in contact with the air. 



In one case however, in which I wanted to introduce air into the tube many 

 times in the same experiment, I used the apparatus represented in fig. 6, con- 

 sisting of a tube ab of a small bore, a ball c, and a tube de of a larger bore. 

 This apparatus was first filled with quicksilver ; and then the ball c and the tube 

 ab were filled with air, by introducing the end a under a glass inverted into 

 water, which contained the proper kind of air, and drawing out the quicksilver 

 from the leg ed by a syphon. After being thus furnished with air, the apparatus 

 was weighed, and the end a introduced into one end of the tube m, and kept 

 there during the experiment; the way of forcing air out of this apparatus into the 

 tube being by thrusting down the tube ed a wooden cylinder of such a size as 

 almost to fill up the whole bore, and by occasionally pouring quicksilver into the 

 same tube, to supply the place of that pushed into the ball c. After the experi- 

 ment was finished, the apparatus was weighed again, which showed exactly how 

 much air had been forced into the tube m during the whole experiment ; it being 

 equal in bulk to a qpantity of quicksilver, whose weight was equal to the increase 

 of weight of the apparatus. 



The bore of the tube m used in most of the following experiments, was about 

 -fLj. of an inch ; and the length of the column of air, occupying the upper part of 

 the tube, was in general from l-i- to 4 of an inch. It is scarcely necessary to in- 

 form any one used to electrical experiments, that in order to force an electrical 

 spark through the tube, it was necessary, not to make a communication between 

 the tube and the conductor, but to place an insulated ball at such a distance from 

 the conductor as to receive a spark from it, and to make a communication be- 

 tween that ball and the quicksilver in one of the glasses, while the quicksilver in 

 the other glass communicated with the ground. I now proceed to the experi- 

 ments. 



When the electric spark was made to pass through common air, included be- 



