COMMUNICATION OF HEAT BETWEEN A SUEEACE AND A GAS. 
735 
suspension more delicate than that then adopted, I was forced to believe that the effect 
found by Mr. Crookes was due to some accidental cause, such as air-currents, about 
the outside of the case of his mill. I therefore repeated Mr. Crookes’s experiment; 
first, by floating the mill, as he describes, in a beaker of water, and simply covering the 
whole with a glass shade. I then found that it was impossible to bring sufficient light 
to bear on the mill to cause the vanes to revolve without causing the case to turn ; 
although this turning was irregular, and such as might be caused by air-currents. 
Dr. Schuster and myself then suspended the same light-mill we had previously used 
in a manner in all respects similar to that of his former experiments, except that the 
mill was upside down, so that the vanes could not turn in the envelope. On the 
light being turned on a certain amount of disturbance was always consequent so long 
as the receiver was not exhausted ; but when the receiver was exhausted to about 
inch of mercury, no motion at all could be observed. At the soiree given by the 
Royal Society on the 14th of June, 1876, I had two mills suspended, the one upright 
and the other reversed. The envelope of the upright mill moved when the light was 
turned on through a distance represented by several hundred divisions of the scale ; 
but the reversed mill showed no motion at all, although a motion of two divisions must 
have been perceived. The mills were suspended in vessels from which the air had been 
pumped until the pressure was about half an inch of mercury. In these experiments, 
therefore, there was no residual force tending to turn the envelope with the mill so great 
as yoq- the force on the mill. 
