140 Mr. Emmett on the [Aug. 



perturbations; it is quite sufficient to know that no motioa 

 ■whatever has been observed in our system which gives the 

 shghtest trace of the operation of any other centripetal force; and 

 if any other existed, the planets, and more particularly the 

 comets, could never describe eUipses, having the sun in one 

 focus. 



In order to explain the phenomena of the expansion and con- 

 traction of solid matter, many have supposed that the particles- 

 of solids are not mutually in contact with each other. Now if 

 they do not touch, they must be preserved at a certain distance 

 from each other, by the equilibrium of two opposite forces ; under 

 these circumstances, they must have perfect freedom of motion 

 among themselves, which is highly inconsistent with the pheno- 

 mena of solidity and cohesion, this freedom of motion of the 

 particles being the primary characteristic of fluidity. By what- 

 ever centripetal force cohesion is supposed to be produced, it 

 can attain only in perfect contact, as is sufficiently proved ia 

 Princip. lib. 1, prop. 81, note 526, by Le Seur and Jacquier, 

 prop. 86, or Simpson's Fluxions, art. 384 : but all solids expand 

 by an increase of their sensible heat, if, therefore, this 

 expansion arises from an absolute separation of the particles 

 from each other, the least possible augmentation of temperature 

 must produce fusion ; besides, since duiiinution of temperature 

 produces contraction, the particles, according to this hypothesis, 

 cannot have previously been in contact, and consequently can- 

 not have had any cohesion, which is absurd. Arguments have 

 been advanced in favour of this hypothesis, deduced i'rom the 

 apparent want of contact between a convex lens and flat plate of 

 glass, although urged by a very considerable force; the inflexion 

 of Hght is quite sufficient to account for the apparent anomaly ; 

 but should this not be allowed, it is easy to prove from other 

 phenomena that they must be in actual contact. While the 

 glasses are strongly pressed together, let the convex lens be 

 moved along the surface of the other glass, and they will 

 mutually scratch each other. In solid matter, the attraction of 

 cohesion is allowed to exceed all the other corpuscular forces ; 

 this, as well as all the other corpuscular forces, is experimentally 

 proved to vanish at the least distance from contact. Now this 

 experiment proves that these pieces of glass do really scratch 

 each other ; i. e. each removes from the surface of the other 

 some of its ultimate atoms ; by which it appears that by some 

 means the attraction of cohesion is overcome ; it cannot have 

 been effected by the agency of the caloric resident in the pieces 

 of glass, because then the repulsive force of caloric in glass must 

 exceed, even at a considerable distance, the force of cohesion, 

 much more then must it exceed it in contact; nor can it have 

 been effected by means of a centripetal force, for then the 

 attraction at a distance must exceed that in contact, or the 

 attractive force vary directly as some power of the distance ; 



