200 OlservaiionS upon the different Hypotheses 



years the sun would lose only the ,3^3.^ part of the matter it 

 now contains. From hence it is evident, that no diminution in 

 the one or increase in the size of the oth?r can as yet, or ever 

 will he, perceived. Dr. Priestley also, when reasoning upon an 

 experiment made by Mitchell, respecting the force of light, has 

 calculated " that in the space of (),(K)0 years there would be 

 only G70 lbs. of the matter of liglit issue from the sun. But 

 little reliance however can he placed upon a calculation founded 

 upon such an exjieviment ; as it is very likely the motion ob- 

 tained bv Mitchell in his machine might be owing to the rare- 

 faction of the air at that point where the rays of light were 

 collected into a focus producing a stream sufficiently strong to 

 put in motion so delicate a piece of machinery. The quantity 

 of heat too, he observes, was so great as to warp the piece of 

 copper considerablv, and almost to melt it; and this would also 

 tend to produce motion, by altering in some measure the centre 

 of gravitv of the instrument. In tiie third place, it has been 

 supposed that the amazing velocity with which the particles of 

 light must move, would, although almost inconceivably minute, 

 have a force exceeding that of a cannon ball. This also Dr. 

 Horslev has ingeniously answered: and he not only proves that 

 the force of a particle of light would not be equal to a cannon 

 shot, but that it will when emitted from the sun, and received 

 directly on the eye, on a bright day, not exceed the force of a 

 ehot a quarter of an inch in diameter, moving at the rate of sixteen 

 inches in a vear ; or in short, the force is less than can be 

 imagined. The celebrated Euler also maintained, that if light 

 were material, its velocity would disturb the motion of the pla- 

 nets. But this is an objection which may also be urged against 

 the doctrine of vibration : for how could the planets move steadily 

 in the midst of a medium that is perpetually in a state of vibra- 

 tion ? This philosopher further urged, that if light consisted of 

 particles passing in straight lines ; then those bodies through 

 which it passes must consist of right-lined canals in ail direc- 

 tions, and that such a structure would deju'ive transparent bodies 

 of all solidity and coherence amongst their parts. This upon 

 investigation does not appear so strong an objection as at first 

 sight. Indeed it is bv no means im]irobable, though undoubtedly 

 hv'pothetical, that transparent substances have their nioleculce or 

 atoms so arranged as to form right lined canals. 



The discoveries made in chemistry since the time of Euler, 

 clearly prove that the ultimate particles of matter are not in ab- 

 solute contact. The expansion and contraction of bodies by the 

 aid of heat sufficiently show this. 



Lavoisier, in the first chapter of his Elements of Chemistry, 

 observes, whea treating upon heat, " that the particles of 



bodies 



