486 



ACCOUNT OF 



and the gas enters to supply its place. The slen- 

 der pipe is then to be sealed hermetically close to 

 the bulb A, by a touch of the flame of a blowpipe. 



The tube A B C is now to be inverted, and the 

 mercury poured out of the cistern C, allowing the 

 column which occupies the tube to run towards the 

 bulb, to prevent the escape of the gas. The tube 

 being again turned into a vertical position, the por- 

 tion of quicksilver which remains is removed, by 

 pouring some of the oil over it, and heating the gas 

 until, by its expansion, it forces the columu of quick- 

 silver which is left at the lower end of the tube, in- 

 to the cistern ; then, holding the tube nearly hori- 

 zontal, the oil will enter as the gas cools, and the 

 remaining quicksilver may be poured out of the cis- 

 tern C. 



The inclosed gas which has thus been introduced, 

 changes its bulk, or occupies more or less space, ac- 

 cording to the pressure of the atmosphere upon the 

 surface of the oil in the cistern C. The scale m n 

 for measuring the change in the bulk of the gas oc- 

 casioned by a change of pressure, is formed experi- 

 mentally, by placing the instrument in an air-tight 

 glass-case, along with an accurate barometer and 

 thermometer. 



The glass-case is furnished with a condensing and 

 exhausting syringe, by which any density may be 

 given to the inclosed gas, so as to support a column 

 of quicksilver in the barometer of 28, 29, 30, or any 

 other required number of inches. The height of 



