1875.) The Mechanical Action of Light. 351 
the quasi vacuum of space we have the radial repulsive force, 
possessing successive propagation, required to account for 
the changes of form in the lighter matter of comets and 
nebulz, and we may learn by that action, which is rapid and 
apparently fitful, to find the cause in those rapid bursts which 
take place in the central body of our system; but until we 
measure the force more exactly we shall be unable to say how 
much influence it may have in keeping the heavenly bodies 
at their respective distances. 
So far as repulsion is concerned, we may argue from 
small things to great, from pieces of pith up to heavenly 
bodies ; and we find that the repulsion shown between a cold 
and warm body will equally prevail, when for melting ice is 
substituted the cold surface of our atmospheric sea in space, 
for a lump of pith a celestial sphere, and for an artificial 
vacuum a stellar void. 
Throughout the course of these investigations I have 
endeavoured to remain unfettered by the hasty adoption of 
a theory, which, in the early stages of an inquiry, must 
almost of necessity be erroneous. Some minds are so con- 
stituted that they seem impelled to form a theory on the 
slightest experimental basis. Thereisthen great danger oftheir 
becoming advocates, and unconsciously favouring facts which 
seem to prove their preconceived ideas and neglecting others 
which might oppose their views. ‘This is unfortunate, for 
the mind should always be free to exercise the judicial 
function, and give impartial weight to every phenomenon 
which is brought it. Any theory will account for some facts; 
but only the true explanation will satisfy al/ the conditions 
of the problem, and this cannot be said of any theory which 
has yet come to my mind. 
My object at present is to ascertain facts, varying the 
conditions of each experiment so as to find out what are the 
necessary and what the accidental accompaniments of the 
phenomena. By working steadily in this manner, letting 
each group of experiments point out the direction for the 
next group, and following up as closely as possible, not only 
the main line of research, but also the little bye-lanes which 
often lead to the most valuable results, after a time the facts 
will group themselves together and tell their own tale; the 
conditions under which the phenomena invariably occur will 
give the laws; and the theory will follow without much 
difficulty. The eloquent language of Sir Humphry Davy 
contains valuable advice, although in terms somewhat 
exaggerated. He says,—‘‘ When I consider the variety of 
theories which may be formed on the slender foundation of 
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