ME. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
547 
as, but in a stronger degree than, it would in a less perfectly exhausted apparatus, viz. 
it was repelled by light and heat of low intensity and attracted by cold. 
A similar experiment was next tried, only water was placed in the bulb before exhaus- 
tion. The water was then boiled away in vacuo , and the exhaustion continued, with 
frequent heating of the apparatus to dull redness, for about forty-eight hours. At the 
end of this time the bar of aluminium was found to behave exactly the same as the 
one in the former experiment, being repelled by radiation. 
Similar experiments, attended with similar results, were tried with a platinum and 
with a glass index ; and instead of water, iodine has been put into the bulb to begin with. 
It is impossible to conceive that in these experiments sufficient condensable gas or 
vapour was present to produce the effects Professor Osborne Reynolds ascribes to it. 
After the repeated heating to redness at the highest attainable exhaustion, it is difficult 
to imagine that sufficient vapour or gas should condense on the movable index to be 
instantly driven off by a ray of light or even the warmth of the finger with recoil enough 
to drive backwards a heavy piece of metal. 
123. It seems tome that a strong argument against Professor Reynolds’s theory (and 
also against the electrical and air-current theories) may be drawn from the fact that the 
repulsion in a vacuum is not confined to those red and ultra-red rays of the spectrum 
which mainly produce dilatation of mercury in a thermometer, excite an electrical 
current between antimony and bismuth couples, and cause a sensation of warmth when 
falling on the skin, but that any ray from the ultra-red to the ultra-violet will produce 
a similar effect. It cannot be reasonably argued that a ray of light, filtered through 
plates of glass and alum (109), can instantly vaporize a film of moisture or condensable 
gas from a surface on which it is caused to shine, or that it can produce air-currents in 
the almost perfect vacuum surrounding the surface shone upon, or that it will produce 
electrical excitement on such a surface. 
124. Facts tested and verified by numerous experiments, but scarcely more than 
touched upon in the present paper, are, I think, gradually shaping themselves in order, 
in my mind, and will, I hope, aid me in evolving a theory which will account for all the 
phenomena. But I wish to avoid giving any theory on the subject until I have accu- 
mulated a sufficient number of these facts. The facts will then tell their own tale ; the 
conditions under which they invariably occur will give the laws ; and the theory will 
follow without much difficulty. In the eloquent words of Sir Humphry Davy, “ When 
I consider the variety of theories which may be formed on the slender foundation of 
one or two facts, I am convinced that it is the business of the true philosopher to avoid 
them altogether. It is more laborious to accumulate facts than to reason concerning 
them ; but one good experiment is of more value than the ingenuity of a brain like 
Newton’s.” 
