ESSENTIAL AND PECULIAR. 55 



glass are within about a ten thousandth part of an inch of each other, using 

 fine metallic plates as a micrometer on this occasion, they support each other's 

 weight as powerfully as if they were in actual contact, and that some additional 

 force is requisite in order to make them approach still nearer. Nor is the 

 force necessary to produce this effect of trivial moment: Professor Robison 

 has calculated it, and has ascertained by experiment that it is equal to a pres- 

 sure of a thousand pounds for every square inch of glass. Air is not neces- 

 sary to this resistance, for it is equally manifest in a vacuum ; yet it is a very 

 curious fact, that under water it almost entirely disappears. It is, however, 

 highly probable that the contact is never perfect, otherwise the two plates 

 might be expected to cohere in such a manner as to become an individual mass. 



It is hence clear that matter, from some cause or other, is possessed of a 

 REPULSIVE as well as of an attractive force ; and that, like the latter, although 

 its law has not been hitherto exactly ascertained, it increases in a regular 

 proportion to its decrease of distance, or, in other words, as bodies approxi- 

 mate each other. 



It has hence been said, and this is the common theory of those who regard 

 gravitation as an essential property of matter, that matter is universally en- 

 dowed with two opposite powers ; by the one of which material substances 

 attract each other, and indnce a perfect union; and by the other of which 

 they repel each other when they are on the point of nnion, and prevent a 

 perfect contact. It is admitted, however, on all hands, and is indeed per- 

 fectly clear in itself, that the repulsive power is of an almost infinitely less 

 range than the attractive. I have supposed the attractive power, or that of 

 gravitation, to operate from world to world ; yet the repulsive power can 

 never be exerted, except " between such particles as are actually, or very 

 nearly, in contact with each other; since it requires no greater pressure, 

 when acting on a given surface, to retain a gallon of air in the space of half 

 a gallon, than to retain a pint in the space of half a pint, which could not pos- 

 sibly be, if the particles exercised a mutual repulsion at all possible distances."* 



This idea, however, of double and opposite powers co-existing in the same 

 substance, and in every corpuscle of the same substance, has been uniformly 

 felt difficult of admission by the best and gravest philosophers ; and hence 

 Sir Isaac Newton, while allowing the repulsive power of matter, which in 

 truth is far more obvious to our senses in consequence of its very limited 

 range, has felt a strong propensity to question gravity as forming an essential 

 property of matter itself, and to account for it from another source. " To 

 show," says he, " that I do not take gravity for an essential property of bodies, I 

 have added one question concerning its cause, choosing to propose it by way of 

 question, because I am not yet satisfied about it, tor want of experiments."! In 

 this question he suggests the existence of an ethereal and elastic medium per- 

 vading all space ; and supports his supposition by strong arguments, and conse- 

 quently with much apparent confidence, deduced from the mediums, or gases, 

 as they are now called, of light and heat, and magnetism, respecting all which, 

 from their extreme subtlety, we can only reason concerning their properties. 

 This elastic medium he conceives to be much rarer within the dense bodies 

 of the sun, the stars, the planets, and the comets, than in the more empty 

 celestial spaces between them, and to grow more and more dense as it 

 recedes from the celestial bodies to still greater distances : by which means 

 all of them, in his opinion, are forced towards each other by the excess of an 

 elastic pressure. 



It is possible, undoubtedly, to account for the effects of gravitation by an 

 ethereal medium thus constituted; provided, as it is also necessary to sup- 

 pose, that the corpuscles of such a medium are repelled by bodies of common 

 matter with a force decreasing, like other repulsive forces, simply as the dis- 

 tances increase. Its density, under these circumstances, would be every 

 where such as to produce the semblance of an attraction, varying like the 

 attraction of gravitation. The hypothesis in connexion with the existence 



* Dr. Young's Lect. vol. i. p. 612. 



t Optics, pref. to the second edition. 



