74 OF SIMPLE ACCELERATING FORCES. 



ther, we conceive that the impenetrability of matter is a 

 suflScient cause for the communication of motion, since the 

 first body cannot continue its course without displacing the 

 second ; and it has been supposed, that if we could dis- 

 cover any similar impulse, which might be the cause of gra- 

 vitation, we should have a perfect idea of its operation. 

 But the fact is, that even in cases of apparent impulse, the 

 bodies impelling each other are not actually in contact ; 

 and if any analogy between gravitation and impulse be ever 

 estabUshed, it will not be by referring them both to the . 

 impenetrability of matter, but to the intervention of some 

 common agent, which must probably be imponderable. It 

 was observed by Newton, that a considerable force was 

 necessary to bring two pieces of glass into a degree of 

 contact, which still was not quite perfect; and Robison 

 has estimated this force at a thousand pounds for every 

 square inch. These extremely minute intervals have been 

 ascertained by observations on the colours of the thin 

 plate of air included between the glasses ; and when an 

 image of these colours is exhibited by means of the solar 

 microscope, it is very easily shown that the glasses are 

 separated from each other, by the operation of this repul- 

 sive force, as soon as the pressure of the screws which 

 confine them is diminished; tho rings of colours, dependent 

 on their distance, contracting their dimensions accord- 

 ingly. Hence it is obvious, that whenever two pieces of 

 glass strike each other, without exerting a pressure equi- 

 valent to a thousand pounds for each square inch, they 

 may affect each other's motion without actually coming 

 into contact. It might perhaps be imagined, that this re- 

 pulsion depended on some particles of air adhering to the 

 glass ; but the experiment has been found to succeed 

 equally well in the vacuum of an air pump. We must. 



