172 Prof. Challis on the II ydrody mimical Theory 



inentary atoms will bo influenced by the position of the 

 affinities mutually satisfied ; or if we prefer to regard the 

 elementary atomic affinities as of equal value, we may still 

 suppose that the mutual action of two elementary atoms will 

 so modify the residuary affinities as to render possible the 

 formation of allotropic forms according to the position in the 

 molecule occupied by a third atom, which now comes within 

 the sphere of action of the molecule. Or it may very pos- 

 sibly be that both of these causes influence the formation of 

 allotropes. We certainly know that the density of some 

 allotropic forms of the same element is not a constant number ; 

 it appears also that changes of energy accompany the changes 

 of an elementary molecule from one allotropic form to 

 another. 



11. Isomerism does not appear to occur among inorganic 

 compounds to so marked an extent as among the compounds 

 of carbon. A variety of silver chloride has been described by 

 Field*, which is unacted upon by light, and is distinctly 

 crystalline ; so, also, we know of two modifications of nitrogen 

 tetroxide, X 2 4 and N0 2 . A systematic search for isomeric 

 bodies among the non-carbon compounds would doubtless meet 

 with a considerable degree of success. 



XXIII. Supplementary Discussion of the Hydrody mimical 

 Theory of Attractive and Repulsive Forces. By Professor 

 Challis, M.A., F.R.S., F.R.A.SJ 



1. TN the mathematical theories of the physical forces which 

 J- from time to time I have proposed and discussed in 

 this Journal, the reasoning has been uniformly made to rest 

 on the following principles : — 



All visible and tangible substances consist of atoms. 



An atom is a very small inert spherical mass of constant 

 form and magnitude. 



All space not occupied by atoms is filled by a perfectly fluid 

 substance (the sether), of uniform pressure and density when 

 at rest, and varying in pressure when in motion in exact pro- 

 portion to variations of its density. 



All physical force acts either as pressure of the gether upon 

 the surfaces of atoms, or as reaction of the atoms upon the 

 aether by reason of the constancy of their form and magnitude. 



2. The selection of these hypotheses has been determined 



* Chem. Soc. Journ. vol. x. p. 242. 

 t Communicated bv the A.uthor, 



