HEAT AND NOT LIGHT A MOTIVE POWER. 
137 
to pass between platinum electrodes. This dazzling light, 
when concentrated by a condensing lens on to a black disc, had 
no effect whatever, although allowed to remain there for many 
minutes. The same spark, when passing between carbon 
electrodes, repelled the black, and produced very slow rotation. 
The carbon was made warm by the electric current, and the 
radiant heat doubtless produced the effect. 
From the experiments I have described, I think we can 
deduce three leading principles which govern the movements of 
radiometers having good reflecting and absorbing discs. 
I. When a radiometer is receiving light or heat, being at a 
lower temperature than its surroundings, repulsion of the black 
discs must ensue, and continue until the temperatures are 
equalised. 
II. When a radiometer is radiating heat, being at a higher 
temperature than its surroundings, attraction of the black discs 
or the apparent repulsion of the white discs, must ensue, and 
continue until the temperatures are again equalised. 
III. No source of light can produce repulsion of the black 
discs of a radiometer unless it is capable of raising the tern 
perature of the residual air within the globe. 
