381 



line perpendicular to that bounding surface, and which is probably 

 attractive towards the liquid or solid, very intense close to the bound- 

 ing surface, but inappreciable at all perceptible distances from it. 

 Such a force can be balanced only by a gradual increase of superfi- 

 cial-atomic elasticity in a direction towards the liquid or solid. Hence, 

 although at perceptible distances from the liquid or solid, the density 

 of vapour is sensibly uniform, the layers close to that surface are 

 probably in a state of condensation by attraction, analogous to that 

 of the earth's atmosphere under the influence of gravity. 



Professor Faraday has expressed an opinion that certain well- 

 known phenomena arise from a state of condensation of this kind, 

 produced in gases by the superficial attraction of various solid sub- 

 stances. 



This third condition of equilibrium is expressed by a differential 

 equation, the integral of which, taken in conjunction with the first 

 two conditions, would be sufficient to determine the respective densi- 

 ties, and the total elasticity of a liquid or solid and its vapour, when 

 in contact with each other in a limited space at any temperature, 

 provided we had a complete knowledge of the laws of molecular force. 

 In the present imperfect state of that knowledge, the integral in 

 question indicates the foitn of an approximate equation, expressing 

 the logarithm of the elasticity of vapour at saturation, in terms of 

 the reciprocals of the first and second powers of the absolute tempe- 

 rature, the coefficients of which the author has calculated empiri- 

 cally, for water and mercury, from the experiments of M. Regnault, 

 and for alcohol, ether, turpentine, and petroleum, fi'oni those of 

 Dr Ure, — three experimental data being required for each fluid, to 

 calculate three constants. The agreement of the results of the 

 formulae thus obtained with those of experiment is as close as the 

 uncertainties of observation render possible, throughout the whole 

 range of pressures and temperatures observed. For steam, in par- 

 ticular, the coincidence is almost perfect. The author gives a table 

 of the constants for the fluids enumerated, and refers to the Edin- 

 burgh New Philosophical Journal for July 18i9, for the details of 

 the comparison between calculation and experiment. 



The section concludes with a speculation as to the probable effects 

 of the atmospheres of dense vapours supposed to exist at the sur- 

 faces of solid and liquid bodies. The author conjectures that the 

 presence of such atmospheres may bo the cause which prevents solid 



