Principles of the Science of Motion. A3 
But such discoveries as that the tangential force of a resisting 
medium is given in the very formule of Encke’s comet*, and 
that of inductive action through contiguous particles, and not at 
a distance}, will, it is believed, cause this first postulate of the 
theory to be generally granted f. 
5. To give distinctness to this idea of the parts of matter as 
mutually repulsive, a molecule, or a body (an aggregate of mo- 
lecules), is conceived as a centre of lines of pressure ; the lengths 
and curves of these lines are determined by the relative pressure 
of the lines they meet ; and lines from greater, are made up of 
lesser molecules and their lines, and so on ad infinitum. 
In speaking of a molecule or body as such a centre of pres- 
sure, it will be convenient to have a technical name. Rather 
than coin a new term, it is proposed to use “atom” in this 
sense. In chemistry I shall use the term “ equivalent ” exclu- 
sively, and not, as at present, as more or less synonymous with 
“atom,” which I have thus ventured to appropriate for a new 
conception. Atoms, or mutually determining centres of lines 
of pressure, may also be defined, and their relations analytically 
investigated, as mutually determining elastic systems with cen- 
tres of resistance. 
6. This is the fundamental conception (not hypothesis) of the 
theory. What can at present be called hypothetical in the 
theory, is this only—that the application of analysis to the con- 
ception of elastic systems with such conditions, will in all cases 
give results corresponding with phenomena. An atom, as above 
defined, is the postulate of a conception, not of an agent, of a 
relation, not of an entity. And this conception, it is believed, 
distinguishes the theory from the many others of which the 
general object has been the same§. Butin this attempt to found 
a general theory cleared of ethers and fluids, of properties and 
virtues, the author owes whatever there may be of truth in its 
present imperfect form and incomplete application, chiefly to 
the discoveries of the immortal ‘ Experimental Researches ;’? and 
the constant endeavour has been to work out the theory in the 
* Ueber die Evxistenz eines widerstehenden Mittels im Weltraume, 
von J. F. Encke, p. 52. Berlin, 1858. 
+ Faraday, ‘ Experimental Researches,’ Series I., II., IV., IX., XI, 
XII., XIII. 
t See also on this point Humboldt, ‘ Cosmos,’ iii. 33; and compare 
Newton’s third letter to Bentley; the old aphorism, “‘ Nature abhors a 
vacuum,” and Grove’s remark thereon, ‘ Correlation of Physical Forces.’ 
And see Bacon, Nov. Org. i. 8. 
§ But compare the conception of an atom by Boscovitch, in his The- 
oria Philosophie Naturalis redacta ad unicam legem virium in Natura 
existentium. Venetiis, 1763; and Faraday, in his ‘ Experimental Re- 
searches,’ ili. 447. Also Seguin’s memoir, Sur l’origine et la propagation 
de la force, aud his Considérations sur les causes de la cohésion. 
