Principles of the Science of Motion. 53 
29. The combination of bodies, in whatever way it takes 
place, and not confining the meaning to the formation of salts*, 
is conceived as the establishment of a new state of dynamic 
equilibrium}. Hence there must necessarily be definite laws of 
- chemical combination. 
But as it is proposed to give the outlines of this mechanical 
theory of chemistry in a future paper or papers, it is unneces- 
sary to enter more fully upon it at present. A theory so im- 
portant has been introduced in a paper in which it must be so 
imadequately expressed, only to complete the general outline of 
the science of motion, and because a mechanical theory of 
chemistry is the necessary complement of the above-given 
mechanical theory of electricity. 
30. In such a theory of forces as that here proposed, the 
great experimental truth of the “ correlation of forces” t assumes 
an axlomaticclearness. specially is it to be noted that it gives 
an account of the difference of the correlation between electricity 
and magnetism, and that of either of these with light, heat, &c. 
The former is a correlation of coexistence, the latter of change 
of conditions. ) 
If I have been successful in making clear the conceptions 
offered in this theory of electricity and magnetism and of 
induction, it will be unnecessary here to say anything further 
of this correlation of coexistence ; for it 1s implied in the concep- 
tion of induction as a mechanical effect varying with resist- 
ance, and manifested at right angles to a longitudinal or trans- 
verse (spiral) line of tension, that is, of permanent (not alterna- | 
ting) molecular displacement. And if electricity is a permanent, 
light an alternating, molecular displacement, their correlation is 
evident, the conditions of their interchange assignable, and ana- 
lytically expressible. 
But not only is the correlation thus clear of physical motions 
among themselves, but also of these, as a class, with mecha- 
nical motions on one side, and chemical motions on the other— 
clear, that is, that the stopping of a mechanical, or the beginning 
of a chemical, motion implies more or less molecular displace- 
ment under such conditions as those above assigned for elec- 
tricity, light, and heat. But what is the proof of such a corre- 
lation of forces but the sublime “ persuasion that all the forces 
* Berthelot’s discoveries have definitively broken down the distinction 
between Inorganic and Organic Chemistry. See his Chimie Organique. 
tT Compare Williamson’s “ Theory of Etherification,” Chem. Soc. Quart. 
Journ. iv. 110. 
{ Grove. See also Helmholtz in Taylor’s Scientific Memoirs, 1853, 
p- 124; and compare Rendu’s “ circulation”’ of fire, light, electricity, and 
magnetism, Théorie des Glaciers de la Savoie, Memoirs of the Royal 
Academy of Sciences of Savoy, 1841, cited in Tyndall’s ‘ Glaciers of the 
Alps,’ p. 299, 
