ON THE PllOPERTIES OF MATTER, 



parent sphere contains in itself, and which is called its centripetal fokce 

 — it will have reached its proper orbit ; and, through the influence of this 

 constant antagonism of the two properties of passivity and gravitation, of £^ 

 centrifugal and centripetal force, persevere in the same to the end of time^ 



Of the immediate cause of gravitation, or the nature of that power 

 which impels different bodies to an union, we are in a very considerable 

 degree of ignorance ; or rather, perhaps, may be said to know nothing at 

 all. It is necessary, however, to notice one very singular phaenomenon 

 concerning it, and to give a glance at two out of various theories, by which 

 gravitation has been attempted to be accounted for. 



The phaenomenon is, that although, owing to this power, all bodies 

 have a tendency to come into contact, they never come into actual con- 

 tact ; some kind of pore or open space being still left between the corpus- 

 cles of bodies that approach the nearest to each other. Thus, a plate of 

 heated iron, solid as it appears to be, and altogether destitute of pores, be- 

 comes contracted in every direction by cold. So, too, as 1 have already 

 observed, equal measures of water and alkohol, or of water and sulphuric 

 acid, have their bulk sensibly diminished. In like manner, Newton has 

 remarked, that when two plates of 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 pro- 

 duce this effect of trivial moment ; Professor Robison has calculated it, 

 and has ascertained by experiment that it is equal to a pressure of about a 

 thousand pounds for every square inch of glass. Air is not necessary to 

 this resistance, for it is equally manifest in a vacuum ; yet it is a very cu- 

 rious 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, hke 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 

 approximate each other. 



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

 gard gravitation as an essential property of matter, that matter is universally 

 endowed with two opposite powers ; by the one of which material sub- 

 stances attract each other and induce a perfect union, and by the other of 

 which they repel each other when they are on the point of union, and pre- 

 vent a perfect contact. It is admitted, however, on all hands, and is in- 

 deed perfectly clear in itself, that the repulsive power is of an almost infi- 

 nitely 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 actu- 

 ally, 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 possibly be, if the particles exercised a mutual repulsion a* 

 all po^ible distances.* 



* Dr. Younac's Lect. vol. i. p. 61?. 



