4$ Dr. Young’s Lecture on 
means of which a motion is produced when they fall on a thin 
plate of copper delicately suspended. (Priestley's Optics.) 
But, taking for granted the exact perpendicularity of the plate, 
and the absence of any ascending current of air, yet since, in 
every such experiment, a greater quantity of heat must be com- 
municated to the air at the surface on which the light falls than 
at the opposite surface, the excess of expansion must necessarily 
produce an excess of pressure on the first surface, and a very 
perceptible recession of the plate in the direction of the light. 
Mr. Bennet has repeated the experiment, with a much more 
sensible apparatus, and also in the absence of air ; and very justly 
infers from its total failure, an argument in favour of the undu- 
latory system of light. (Phil. Trans, for 1792, p. 87.) For, 
granting the utmost imaginable subtility of the corpuscles of 
light, their effects might naturally be expected to bear some 
proportion to the effects of the much less rapid motions of the 
electrical fluid, which are so very easily perceptible, even in 
their weakest states. 
3. There are some phenomena of the light of solar phosphori, 
which at first sight might seem to favour the corpuscular sys- 
tem ; for instance, its remaining many months as if in a latent 
state, and its subsequent re-emission by the action of heat. 
But, on further consideration, there is no difficulty in supposing 
the particles of the phosphori which have been made to vibrate 
by the action of light, to have this action abruptly suspended 
by the intervention of cold, whether as contracting the bulk of 
the substance or otherwise; and again, after the restraint is 
removed, to proceed in their motion, as a spring would do which 
had been held fast for a time in an intermediate stage of its vibra- 
tion ; nor is it impossible that heat itself may, in some circum- 
stances*. become in a similar manner latent. (Nicholson’s 
