1844.] of Bodies generally existing as Gases. 



ing substances, namely, water, sulphurous acid, cyanogen, am- 

 monia, arseniuretted hydrogen, sulphuretted hydrogen, muri- 

 atic acid, carbonic acid, olefiant gas, &c., justify any conclusion 

 respecting a general law, it would appear that the more volatile 

 a body is, the more rapidly does the force of its vapour in- 

 crease by further addition of heat, commencing at a given point 

 of pressure for all ; thus for an increase of pressure from two 

 to six atmospheres, the following number of degrees require to 

 be added for the different bodies named : water 69, sulphurous 

 acid 63, cyanogen 64*5, ammonia 60, arseniuretted hydro- 

 gen 54, sulphuretted hydrogen 56*5, muriatic acid 43, 

 carbonic acid 32'5 t nitrous oxide 30; and though some of 

 these numbers are not in the exact order, and in other cases, 

 as of olefiant gas and nitrous oxide, the curves sometimes even 

 cross each other, these circumstances are easily accounted for 

 by the facts already stated of irregular composition and the 

 inevitable errors of first results. There seems every reason 

 therefore to expect that the increasing elasticity is directly as 

 the volatility of the substance, and that by further and more 

 correct observation of the forces, a general law may be deduced, 

 by the aid of which, and only a single observation of the force 

 of any vapour in contact with its fluid, its elasticity at any other 

 temperature may be obtained. 



Whether the same law may be expected to continue when 

 the bodies approach near to the Cagniard de la Tour state is 

 doubtful. That state comes on sooner in reference to the press- 

 ure required, according as the liquid is lighter and more expan- 

 sible by heat and its vapour heavier, hence indeed the great 

 reason for its facile assumption by ether. But though with 

 ether, alcohol and water, that substance which is most volatile 

 takes up this state with the lowest pressure, it does not follow 

 that it should always be so ; and in fact we know that ether 

 takes up this state at a pressure between thirty-seven and thirty- 

 eight atmospheres, whereas muriatic acid, nitrous oxide, car- 

 bonic acid and olefiant gas, which are far more volatile, sustain 

 a higher pressure than this without assuming that peculiar state, 

 and whilst their vapours and liquids are still considerably dif- 

 ferent from each other. Now whether the curve which ex- 

 presses the elastic force of the vapour of a given fluid for in- 

 creasing temperatures continues undisturbed after that fluid 



