484 M. R. Clausius on the Tension and the Latent Heat 



hence the various efforts which have heen made to ascertain a 

 definite law by means of which the series which holds good for* 

 one fluid, water for instance, might be applied to other fluids. 



A veiy simple law of this nature was expressed by Dalton. 

 Calling those temperatures which belong to equal tensions cor- 

 respoiiding temperatures, the law ran thus : — In the case of any 

 two fluids the differences between the corresponding temperatures 

 are all equal, 



Tliis law agrees pretty well with experience in the case of 

 those fluids whose boiling-points are not far apart; for those, 

 however, which possess very different degrees of volatility it is 

 inexact. This is shown by a comparison of the vapour of mercury 

 with that of water, according to the observations of Avogi-ado*. 

 Still more decidedly does the divergence exhibit itself in the in- 

 vestigations of Faradayt on the condensation of gases. 



In the '^ Additional Remarks '' to his memoir, Mr. Faraday, 

 after having disproved the applicabihty of the law of Dalton to 

 gases, expresses himself as follows : — ^' As far as observations 

 upon the following substances, namely, water, sulphurous acid, 

 cyanogen, ammonia, arseniuretted hydrogen, sulphuretted hy- 

 drogen, 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 its vapour increase 

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

 pressure for all;" and further on, "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." 



What Faraday here expresses with evident reserve and caution, 

 we find again in the form of an equation in a later memoir by 

 M, GroshansJ, The equation (3.) of the said memoir contains 

 implicitly the following law ; — If all temperatures from --273'' C. 

 downwards (that is, downwards from that temperature which is 

 expressed by the inverse value of the coefficient of expansion for 

 atmospheric air) be reckoned^ then for any two fluids the corre^ 

 sponding temperatures are proportional, 



Although this carries with it a great degree of probability, at 

 least as an approximate law, and is undoubtedly proved by the 



* Ann. de Chim. et de Phys. xlix. p. 369. Pogg. 4nn. vol. jcxvii. p. 60. 

 Complete in M^. de VAcad. de Turin, vol. xxxvi. 

 t Phil. Trans, of the Roy. Soc. of London for 1845, p. 165. 

 j Pogg. Ann. vol. Uxviii. p. 112. 



