282 



bodies from cohering when brought together, and produces that re- 

 sistance to contact which is visible not only in them but in drops of 

 liquid. He conceives it possible that it may also be the cause of 

 the " spheroidal state" of liquids at high temperatures, and may 

 assist in maintaining the vesicular state, if such a state exists. 



The sixth and last section of the first part relates to mixtures of 

 gases and vapours of different kinds. 



The principle stated in the second section that the elasticity of 

 the atomic atmosphere is proportional to its density, is here expressed 

 in the form, that the elasticity of any numher of portions of atomic 

 atmosphere, compressed into a given space, is equal to the sum of the 

 elasticities which such p>ortions would respectively have if they occu- 

 pied the same space separately . It is shewn, that if this principle 

 be considered true, not only of portions of atomic atmosphere of one 

 kind of substance, but also of portions of atomic atmospheres of sub- 

 stances of different kinds, when mixed, it leads to the well-known 

 laws of the elasticity and diffusion of mixed gases and vapours. He 

 also speculates on the possibility of solid bodies, which have no per- 

 ceptible vapours of their own at ordinary temperatures, acquiring 

 the power of resisting cohesion by means of a superficial atmosphere 

 of foreign substances. 



The second part of the paper treats of the dynamical relations of 

 the heat and the elasticity of bodies in the gaseous state. 



The first section contains the general theory of the mutual con- 

 version of heat and expansive power. 



After recapitulating the mode of expressing quantities of heat in 

 terms of gravity, the author refers to the experiments of Mr] Joule 

 on the production of heat by electro-magnetic currents, by friction, 

 and by the compression of air, as proving the convertibility of heat 

 and mechanical power. He states reasons, however, for believing 

 that the mechanical value of heat as deduced from those experiments 

 (viz., from 760 feet to 890 feet per degree of Fahrenheit, applied to 

 liquid water) is too large, owing to various causes of loss of power, 

 and gives the preference to experiments in which no machinery is 

 used, such as those on the velocity of sound, as data for such a cal- 

 culation. 



The laws of the production of heat by compression, and its con- 

 sumption by expansion, are then deduced from the following two 

 principles, the first of which is peculiar to the hypothesis of mole- 



