V 



ESSENTIAL AND PECULIAR^ 4^ 



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 

 hmited 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 1 do not take gravity for an essential property 

 of bodies, 1 have added one question concerning its cause, choosing to 

 propose it by way of question, because I am not yet satisfied about it, for 

 want of experiments.^'* In th)s question he suggests the existence of an 

 ethereal and elastic medium pervading all space ; and supports his suppo- 

 sition by strong arguments, and consequently with much apparent con- 

 fidence, deduced from the mediums, or gases, as they are now called, of 

 light and heat, and magnetism, respecting all which, from their extreme 

 subtilty, 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 eflfects of gravitation by 

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

 suppose, 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 distances 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 of a repulsive force in common matter has a great advantage in 

 point of simplicity, and may perhaps hereafter be capable of proof, though 

 at present it can only be regarded, and was at first only offered, as an 

 hypothesis. 



M. La Place, equally dissatisfied as Sir Isaac Newton with the idea of 

 gravitation being an essential property of matter, passes awaf from the 

 inquiry with suitable modesty, to practical subjects of far higher import- 

 ance, and which equally grow out of it, in whatever light it is contemplated. 

 " Is this principle," says he, a primordial law of nature ? oris it a general 

 effect of an unknown cause ? Here we are arrested by our ignorance of 

 the nature of the essential properties of matter, and deprived of all hope of 

 answering the question in a satisfactory manner. Instead, then, of forming 

 hypotheses on the subject, let us content ourselves with examining more 

 particularly the manner in which philosophers have made use of this most 

 extraordinary power."! 



There is, indeed, one very striking objection to Sir Isaac Newton's sug- 

 gestion, and which it seems very difficult to repel. It is, that though it 

 may account for the attraction of gravitation, as a phasnomenon common 

 to matter in general, it by no means accounts for a variety of particular 

 attractions which are found to take place between particular bodies, or 

 bodies particularly circumstanced ; and which, excepting in one or two 



* Optics, pref. to the second edition. 



t Exposition du Systeme du Monde, lib. iv. eh. xv. 



1 



