Principles of Energetics. 351 
nation of phenomena. Previous to such an attempt, it is thought 
advisable to enunciate these principles in their most general form, 
and give them merely experimental illustration. 
The principles to be set forth in this paper will lead me to 
remark on the physical theories recently published in this Journal 
by Prof. Challis and Prof. Maxwell. It will be found that as my 
theory refers attractions to differential conditions of stress and 
strain, of pressure and tension, among elastic bodies, it agrees 
rather with the molecular theory of the latter, than with the hy- 
drodynamical theory of the former; that the point of funda- 
mental difference from both is in the conception offered of 
Matter; and that on this point my theory is a development of 
the views to which experiment has led Mr, Faraday. 
17. (I.) Atoms are mutually determining centres of pressure. 
18. If this idea of an atom, as a body of any size, acting and 
reacting on another similar body by the pressure of the con- 
tinued, infinitesimal, but similar particles of which each centre 
is an aggregate, be clearly conceived, it may be expressed in 
many different forms. I have, for instance, in the introductory 
paper spoken of a body thus conceived as a Centre of Lines of 
Pressure, or an Elastic System with a centre of resistance. But 
here, more clearly to express the idea in contrast with the fun- 
damental hypotheses of Prof. Challis, an atom may be defined 
as a centre of an emanating elastic ether, the pressure of which 
is directly as the mass of its centre, and the form of which de- 
pends on the relative pressures of surrounding atoms. Thus, if 
you will, matter may be said to be made up of particles in an 
elastic ether. But that ether is not a uniform circumambient 
fluid, but made up of the mutually determining ethers (if you 
wish to give the outer part of the atom a special name) emana- 
ting from the central particles. And these central particles are 
nothing but what (endeavouring to make my theory clear by 
expressing it in the language of the theories it opposes) I may 
call zetherial nuclei. 
“ Hence,” according to the conception of Faraday, “ matter 
will be continuous throughout, and in considering a mass of it 
we have not to suppose a distinction between its atoms and any 
intervening space....... The atoms may be conceived of as 
highly elastic, instead of being supposed excessively hard and 
unalterable in form..... With regard also to the shape of the 
SLOMS . 6. ee That which is ordinarily referred to under the 
term shape would now be referred to the disposition and relative 
intensity of the forces *.” 
* Phil. Mag. 1844. vol. xxiv. p. 142; or Experimental Researches, 
vol. ii. p. 284, See also Phil. Mag. 1846, vol. xxviii. No, 188; or Expe- 
rimental Researches, vol. iii. p. 447. 
