Prineiples of the Science of Motion. 49 
displacement and the varying resistance offered by the diverse 
conditions of motion and aggregation of opposed centres of lines 
of pressure. 
22. Now, as to the facts which justify the proposal of this 
conception as a true generalization by which mechanical principles 
ean be immediately applied to the phenomena of induction, it 
may be remarked that, as the full proof of such a conception 
depends on an experiraentally based, and analytically expressed, 
mechanical theory of the constitution of bodies, it is only sur- 
prising that, while so little has been done towards the establish- 
ment of such a theory, the general conclusions of researches on 
induction, independent of, or with wrong, theories of chemical 
constitution, should go so far to make induction mechanically 
conceivable. Among such conclusions, each of which will call 
up avast number of experimental facts, may be mentioned :— 
Induction is the origin and effect of all electrical phenomena; 
and is an action, not “at a distance,” but “through contiguous 
particles”’ in lines of any curve. Insulation and discharge, or 
conduction, are differences only of degree; and bodies have 
specific mductive capacities which are but degrees of resisting 
power. The degree to which particles are affected before dis- 
charge constitutes intensity; and; in order to discharge, inten- 
sity must be raised much higher for a solid than for a fluid, and 
higher for a fluid, than for a gaseous, dielectric. An electric 
current has not only polar, but lateral, inductive effects; and 
the “lines of force” about a magnet take the form of a 
“sphondyloid.” As to such facts as that gases, having the 
same inductive capacity, differ in insulating power, and that the 
effect of a magnet on a body without it is not affected by great 
rarefication of the medium (or a vacuum), their explanation more 
particularly depends on a mechanical theory of the constitution 
of bodies, and the principle that the character of inductive effect 
depends on the conditions of molecular motion and aggregation 
of the body acted on. 
23. The general principle by which this theory gives a common 
explanation of the above classified (B) («) motions of bodies in 
presence of an electrified or magnetized body, is that the mechant- 
cal motions of bodies in such circumstances are effects of differ- 
ential molecular displacement ; or, as it may be otherwise ex- 
pressed, if, of two bodies, one resists molecular displacement 
from a centre of disturbance less than the other, the former 
moves towards that centre as a direction of least resistance. 
For it is evident that if a force has its full effect in a molecular 
displacement, the body will, as far as the direct action of that 
force is concerned, remain at rest; and that if the molecules of 
a body resist displacement, the force will have its effect in the. 
Phil. Mag. 8. 4. Vol. 21. No, 187. Jan. 1861. 1D 
