82 Mr. T. Graham's Speculative Ideas respecting 



motion of an atom being inalienable, light matter is no longer 

 convertible into heavy matter. In short, matter of different 

 density forms different substances — different inconvertible ele- 

 ments as they have been considered. 



What has already been said is not meant to apply to the 

 gaseous volumes which we have occasion to measure and practi- 

 cally deal with, but to a lower order of molecules or atoms. The 

 combining atoms hitherto spoken of are not therefore the mole- 

 cules the movement of which is sensibly affected by heat with 

 gaseous expansion as the result. The gaseous molecule must 

 itself be viewed as composed of a group or system of the pre- 

 ceding inferior atoms, following as a unit laws similar to those 

 which regulate its constituent atoms. We have indeed carried 

 one step backward, and applied to the lower order of atoms, ideas 

 suggested by the gaseous molecule, as views derived from the 

 solar system are extended to the subordinate system of a planet 

 and its satellites. The advance of science may further require 

 an indefinite repetition of such steps of molecular division. The 

 gaseous molecule is then a reproduction of the inferior atom on 

 a higher scale. The molecule or system is reached which is af- 

 fected by heat, the diffusive molecule, the movement of which is 

 the subject of observation and measurement. The diffusive 

 molecules are also to be supposed uniform in weight, but to 

 vary in velocity of movement, in correspondence with their con- 

 stituent atoms. Accordingly the molecular volumes of different 

 elementary substances have the same relation to each other as 

 the subordinate atomic volumes of the same substances. 



But further, these more and less mobile or light and heavy 

 forms of matter have a singular relation connected with equality 

 of volume. Equal volumes of two of them can coalesce together, 

 unite their movement, and form a new atomic group, retaining 

 the whole, the half, or some simple proportion of the original 

 movement and consequent volume. This is chemical combina- 

 tion. It is directly an affair of volume, and only indirectly 

 connected with weight. Combining weights are different because 

 the densities, atomic and molecular, are different. The volume 

 of combination is uniform, but the fluids measured vary in den- 

 sity. This fixed combining measure — the metron of simple sub- 

 stances — weighs 1 for hydrogen, 16 for oxygen, and so on with 

 the other " elements." 



To the preceding statements respecting atomic and molecular 

 mobility, it remains to be added that the hypothesis admits of 

 another expression. As in the theory of light we have the alter- 

 native hypotheses of emission and undulation, so in molecular 

 mobility the motion may be assumed to reside either in sepa- 

 rate atoms and molecules, or in a fluid medium caused to undu- 



