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XXVIII. On the Forces caused by the communication of Heat between a Surface and a 
Gas ; and on a new Photometer. By Prof. Osborne Reynolds. Communicated by 
B. Stewart, F.B.S., Professor of Natural Philosophy in Owens College, Manchester. 
Received February 24, — Read March 23, 1876. 
In a paper read before the Royal Society*, April 1874, I pointed out that the commu- 
nication of heat from a solid surface to a gas, whether accompanied by evaporation or 
not, must, according to the kinetic theory, be attended by a reactionary force equi- 
valent to an increase in the pressure of the gas on the surface, and, conversely, when 
heat is communicated from the gas to the surface the pressure against the surface is 
diminished ; and I also suggested that these forces are the probable cause of the motion, 
resulting in some way from radiation, which Mr. Crookes had brought into such pro- 
minent notice. 
Since the publication of this paper neither my conclusions as to the existence of 
these “heat reactions,” nor the reasoning by which I supported them, have been contro- 
verted or even questioned; but, on the other hand, they have received important 
confirmation. The results at which Professors Tait and Dewar arrived after a careful 
investigation fully bear out my conclusions, not only as to the existence of the forces, 
but also as to the way in which they explain Mr. Crookes’s experiments. 
Still it seemed desirable, if possible, to settle the question by obtaining such quan- 
titive measurements of the effects produced as would show whether or not they agreed 
with what might be expected from theoretical considerations. I have accordingly been 
on the look-out for some means of making these experimental verifications. Such a 
means I at length found in one of the beautiful little instruments constructed by 
Dr. Geissler, of Bonn, after the manner of Mr. Crookes, and called by him “ Light- 
Mills.” As this instrument has taken an important part in the experiments I have to 
describe, I shall commence by giving a detailed description of it. 
The Light-Mill. 
This consists of a glass envelope in the shape qf a pear, about 2\ inches through its 
thickest part; standing up from its lower end is a steel needle, coincident with its 
axis. On the top of this needle is balanced (after the manner of a compass-card) the 
mill or wheel ; this consists of a small central glass cup which rests on the point of the 
needle, and to which are fused four very thin platinum arms, which have their outer 
ends attached to four square plates (which appear to be talc or mica-schist) 4 inch 
* Roy. Proc. 1874, vol. xxii. p. 401. . 
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MDCCCLXXVI, 
