840 M. V. llegnault on some Apparatus for determining 



pressures II, H', II", and the spaces V, V, V" noted which it 

 occupies in the tube AB. The lowest pressure under which this 

 apparatus could be worked would be that for which mercury 

 would stop at 1) in the tube CD, and the highest would be that 

 in which the mercury would stand at the top C of this tube. 

 These limits might be indefinitely increased by connecting C D 

 with a reservoir W, in which the air is either exhausted or com- 

 pressed. The elastic force of the air in this reservoir is measured 

 by a barometric pressure-gauge (fig. 15) when the air is expanded, 

 or with an ordinary manometer when it is compressed. The 

 mercury may also be kept at the same level in both tubes A B 

 and CD. 



These experiments may be repeated by raising the water in the 

 bath successively to gradually higher temperatures, and keeping 

 the temperature constant during each series of determinations. 

 There are thus all the elements necessary for knowing the law of 

 the compressibility of the gas for various temperatures. 



The same experiments give all the data necessary for calcula- 

 ting the coefficient of expansion of gases under different pres- 

 sures. The experiments may be made so as to determine the 

 real expansion of the gas, the latter being always under the same 

 pressure at different temperatures; or so as to measure the 

 change which the elastic force of the gas undergoes for variations 

 of temperature, its volume remaining the same. 



To deduce from these experiments the density of a gas under 

 different pressures and at different temperatures, its weight must 

 be known. For this purpose different means may be employed, 

 according to the chemical properties of the gas. The most 

 general method consists in having a globe provided with a stop- 

 cock, which, by means of a capillary tube of platinum, silver, or 

 copper, can be exactly fitted on the prolongation b m of the tube 

 AB. Vacuum having been produced in this globe, the gas con- 

 tained in the tube A B is passed into it, and the globe again 

 weighed by the method of compensating-weights. The increase 

 in the w T eight of the globe gives the weight of the gas. The gas 

 can frequently be absorbed directly by chemical agents — for 

 instance, carbonic acid, sulphurous acid, sulphuretted hydrogen, 

 ammonia, &c. In this case the tube b m is connected with an 

 apparatus containing the absorbing substance; the weight of the 

 gas is indicated by the increase in weight of the absorbing sub- 

 stance. Lastly, in many cases the weight of gas may be deter- 

 mined by chemical analysis : thus, for the carburetted hydrogens 

 and the various gases of organic chemistry, the gas from the 

 tube AB is made to pass through an apparatus for organic 

 analysis. 



The same apparatus can be readily used for vapours. The 



