522 
The Philosophy of the Radiometer. [October, 
proved to produce our sensations, and the other phenomena 
of sound. On this basis there is no difficulty in conceiving 
the rationale of the reaction which produces the repulsion of 
the radiometer. I may even go further, and affirm that with 
this basis to our philosophy, it is impossible to rationally 
conceive radiation without mechanical reaction. If heat be 
motion, and actual motion of aCtual matter, mechanical 
force must be exerted to produce it, and a body which is 
warmer on one side than the other, i.e., which is exerting 
more outward motion-producing force on one side than on 
the other, must be subject to proportionally unequal reaction, 
and therefore, if free to move, must retreat in a direction 
contrary to that of its greater activity. Regarded thus the 
residual air of the radiometer does a Ct, not by collisions of 
particles between the vane and inside of the glass vessel, but 
by the direct reaction of the radiant energy which would 
operate irrespective of vessels, i.e., upon naked radiometer 
vanes if carried half way to the moon, or otherwise freed 
from excess of atmospheric embarrassment. 
The recent experiments of Mr. Crookes, showing retarda- 
tion of the radiometer with extreme exhaustion, seem to 
indicate that heat-rays, like the eleCtric discharge, demand 
a certain amount of atmospheric matter as their carrier. 
I cannot conclude these hasty and imperfeCt notes, 
written merely with suggestive intent, without quoting a 
passage from the preface to the “ Correlation of Physical 
Forces,” which, though written so long ago, appears to me 
worthy of the profoundest present consideration. 
“ It appears to me that heat and light may be considered 
as affeCtions ; or, according to the undulatory theory, vibra- 
tions of matter itself, and not of a distinct ethereal fluid 
permeating it : these vibrations would be propagated just as 
sound is propagated by vibrations of wood or as waves by 
water. To my mind all the consequences of the undulatory 
theory flow as easily from this as from the hypothesis of a 
specific ether ; to suppose which, namely, to suppose a fluid 
sui generis and of extreme tenuity penetrating solid bodies, 
we must assume, first, the existence of the fluid itself ; 
secondly, that bodies are without exception porous ; thirdly, 
that these pores communicate ; fourthly, that matter is 
limited in expansibility. None of these difficulties apply to 
the modification of this theory which I venture to propose ; 
and no other difficulty applies to it which does not equally 
apply to the received hypothesis.” 
