280 



grade degree), when the volume and pressure at that tempera- 

 ture are respectively taken as units. The state of perfect gas, how- 

 ever, can be only approximated to in nature ; for in all gases, espe- 

 cially the more dense and composite, the actions of the atomic nuclei 

 or centres on their atmospheres, and of separate atoms upon each 

 other, have more or less influence on the elasticity. 



M. Regnault has made several elaborate series of experiments, to 

 determine the deviations from uniform expansibility thus produced, 

 in various gases. 



The author, by applying his theory to data furnished by the ex- 

 periments of M. Regnault, has obtained formulae for the coefficients 

 of expansion of atmospheric air, carbonic acid gas, and hydrogen, 

 the results of which agree closely with those of observation, in every 

 case in which a comparison is possible. 



The fifth section treats of the elasticity of vapour in contact with 

 the same substance in the liquid or solid state, or what is called the 

 pressure of vapour at saturation. 



The equilibrium of a substance filling a limited space, partly in 

 the form of vapour, and partly in that of liquid or solid, is shewn to 

 depend on three conditions. 



The first condition of equilibrium is, that the total elasticity of 

 the substance in the two states must be the same. 



The second condition of equilibrium is, that the superficial elasti- 

 cities of every two contiguous atoms must be the same at their sur- 

 face of contact, and hence, that the superficial-atomic elasticity must 

 vary continuously ; so that, if, at the bounding surface between the 

 liquid or solid and its vapour, there is an abrupt change of density, 

 (as the reflection of light renders probable) there must there be two 

 densities corresponding to the same superficial atomic elasticity. 



The third condition of equilibrium is deduced from the mutual at- 

 tractions and repulsions of the atoms of liquid or solid and those of 

 vapour. In a gas in which the atomic centres are equidistant, the 

 actions of the several atoms on each individual particle at an appre- 

 ciable distance from the bounding surface of the gas, balance each 

 other, and are accordingly treated as merely affecting the total elas- 

 ticity by a quantity which is a function of the density ; but near the 

 bounding surface between a liquid or solid and its vapour, the action 

 of the liquid or solid upon any atom must be greater than that of the 

 vapour. A force is thus produced which acts on each particle in a 



