544 
ME. W. CEOOKES ON EEPTTLSION EES TILTING FEOM EADIATION. 
move 46-8 divisions. These experiments were, however, tried in 1873*, when I had 
not succeeded in getting any thing like the delicacy I now obtain in the apparatus ; and 
I propose to repeat them under varied conditions before employing the results to 
found any arguments upon. 
117. In my former paper on this subject (74, 75, 76, 77, 78) I have discussed various 
explanations which may be given of attraction and repulsion resulting from radiation ; 
and in a lecture delivered before the Physical Society f I entered more fully into the 
same arguments. The most obvious explanation is that the movements are due to the 
currents formed in the residual gas, which, theoretically, must be present to some extent 
even in those vacua which are most nearly absolute. 
Another possible explanation is that the movements are due to electricity developed 
on the moving body, or on the glass apparatus, by the incident radiation. 
A third explanation has been put forward by Professor Osborne Reynolds, in a paper 
which was read before the Royal Society on June 18th, 1874. Referring to the results 
of my experiments, Professor Reynolds says that it is the object of his paper to prove 
that these effects are the result of evaporation and condensation. In my exhausted 
tubes he assumes the presence of aqueous vapour, and then argues as follows : — “ When 
the radiated heat from the lamp falls on the pith, its temperature will rise, and any 
moisture on it will begin to evaporate and to drive the pith from the lamp. The 
evaporation will be greatest on that ball which is nearest to the lamp ; therefore this 
ball will be driven away until the force on the other becomes equal, after which the 
balls will come to rest, unless momentum carries them further. On the other hand, 
when a piece of ice is brought near, the temperature of the pith will be reduced, and it 
will condense the vapour and be drawn towards the ice.” 
It is not my intention to recapitulate the arguments I have already brought forward 
against these three explanations. They are all fully given in my above-quoted lecture 
before the Physical Society. I shall, however, adduce a few experiments which have 
been devised specially with the view of putting one or other of these theories to the 
test. In giving what I conceive to he reasonable arguments against the explanations 
which have already been proposed, I do not, however, wish to insist upon any theory 
of my own to take their place. Any theory will account for some facts ; but only the 
true explanation will satisfy all the conditions of the problem, and this cannot be said 
of either of the theories I have already discussed. 
118. The pendulum-apparatus, described and figured in paragraph 99, was specially 
devised to bear upon the air-current and the electrical theory. On referring to the 
description of the experiments tried with it (Tables I. & II.), it is seen that in air the 
ignited spiral produced attraction, whilst in a vacuum the same source of radiation gave 
* The torsion-apparatus with ivory terminals was exhibited in action at the Meeting of the Eoyal Society, 
Dec. 11th, 1873. 
t Tune 20, 1874 (Phil. Mag., August 1874). 
