Compression of Air and other Gases. 231 



In these tables, the first column shows the weight of air in- 

 troduced into the reservoir ; the second shows the condensa- 

 tion ; the third the force required to open the valve, diminish- 

 ed by the force required to open it before the charge ; and the 

 fourth gives the pressure of the atmosphere obtained by di- 

 viding this force by the condensation. 



In the first table, the mean number is 797 ; the deviations 

 from it vary on both sides. In the second table, we have, 

 by rejecting the first number as too discrepant, 1027 as 

 the mean, and it is evident that most of the numbers are not 

 far removed from it. These experiments, therefore, imper- 

 fect as they are by their very nature, concur in proving that 

 the compressions produced by very great forces, are regulat- 

 ed by the same law as those which are feeble. But, in or- 

 der to decide if the compressions of any gas whatever follows 

 the same law, we have had recourse to gases capable of being 

 liquefied by a few atmospheres. The sulphurous acid gas, 

 which, according to M. Faraday, is liquefied by a pressure of 

 two atmospheres, appeared to us very fit for this class of ex- 

 periments. 



Two equal tubes, the one filled with sulphurous acid very 

 dry, and the other with atmospheric air, were placed in a 

 small mercurial bath, and introduced into an apparatus, where 

 these aeriform bodies could be exposed to a suitable pres- 

 sure ; the result was, that they suffered a diminution of 

 their volume always equal, till the sulphurous acid gas began 

 to convert itself into a liquid. The following is the detail of 

 the experiment. 



In Fig. 18, A AAA is a cylinder of glass very strong, and 

 of the same kind as that which I used to show the compres- 

 sion of water. This cylinder has a brass cover surmounted 

 with a cylinder BBBB, in which moves the piston C, pushed 

 by the screw DD ; EE, EE are two equal and divided tubes, 

 whose lower ends are immersed in an iron dish FF, which is 

 fixed to the end of a strip of glass GGGG, which, at the 

 same time, serves to keep the two tubes in a vertical position. 

 The cylinder AAAA is filled with mercury to HH. The 

 experiment is begun by filling the two tubes with the aeriform 

 materials, placing them in the dish FF, and fixing them on 

 the strip of glass GGGG. When this is done, the apparatus 



