176 DR. FARADAY ON THE LIQUEFACTION AND SOLIDIFICATION OF 



From the former scanty and imperfect expressions of the elasticity of the vapour 

 of the condensed gases, Dove was led to put forth a suggestion*, whether it might not 

 ultimately appear that the same addition of heat (expressed in degrees of the ther- 

 mometer) caused the same additional increase of expansive force for all gases or 

 vapours in contact with their liquids, provided the observation began with the same 

 pressure in all. Thus to obtain the difference between forty-four and fifty atmospheres 

 of pressure, either with steam or nitrous oxide, nearly the same number of degrees of 

 heat were required ; to obtain the difference between twenty and twenty-five atmo- 

 spheres, either with steam or muriatic acid, the same number were required. Such a 

 law would of course make the rate of increasing expansive force the same for all bodies, 

 and the curve laid down for steam would apply to every other vapour. This, however, 

 does not appear to be the case. That the force of the vapour increases in a geometrical 

 ratio for equal increments of heat is true for all bodies, but the ratio is not the same 

 for all. As far as observations upon the following substances, namely, water, sul- 

 phurous acid, cyanogen, ammonia, arseniuretted hydrogen, sulphuretted hydrogen, 

 muriatic 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 increase 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 hydrogen 

 54°, sulphuretted hydrogen 56°"5, muriatic acid 43°, carbonic acid 32°o, 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 pressure required, according as the liquid is lighter and more expansible 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 be- 

 tween thirty-seven and thirty-eight atmospheres, whereas muriatic acid, nitrous oxide, 

 carbonic acid and olefiant gas, which are far more volatile, sustain a higher pressure 

 * Poggendorff's Annalen, xxiii. 290 ; or Thomson on Heat and Electricity, p. 9. 



