4G8 LECTURE XLIX. 



in the solar system any considerable quantity of a substance even so dense 

 as platina. 



Besides this porosity, there is still room for the supposition, that even the 

 ultimate particles of matter may be permeable to the causes of attractions 

 of various kinds, especially if those causes are immaterial : nor is there 

 anything in the unprejudiced study of physical philosophy that can induce 

 us to doubt the existence of immaterial substances ; on the contrary we see 

 analogies that lead us almost directly to such an opinion. The electrical 

 fluid is supposed to be essentially different from common matter; the 

 general medium of light and heat, according to some, or the principle of 

 caloric, according to others, is equally distinct from it. We see forms of 

 matter, differing in subtility and mobility, under the names of solids, 

 liquids, and gases ; above these are the semimaterial existences which 

 produce the phenomena of electricity and magnetism, and either caloric or 

 a universal ether ; higher still perhaps are the causes of gravitation, and 

 the immediate agents in attractions of all kinds, which exhibit some phe- 

 nomena apparently still more remote from all that is compatible with 

 material bodies ; and of these different orders of beings, the more refined 

 and immaterial appear to pervade freely the grosser. It seems therefore 

 natural to believe that the analogy may be continued still further, until it 

 rises into existences absolutely immaterial and spiritual. We know not 

 but that thousands of spiritual worlds may exist unseen for ever by human 

 eyes ; nor have we any reason to suppose that even the presence of matter, 

 in a given spot, necessarily excludes these existences from it. Those who 

 maintain that nature always teems with life, wherever living beings can be 

 placed, may therefore speculate with freedom on the possibility of indepen- 

 dent worlds ; some existing in different parts of space, others pervading 

 each other, unseen and unknown, in the same space, and others again to 

 which space may not be a necessary mode of existence. 



Whatever opinion we may entertain with respect to the ultimate impe- 

 netrability of matter in this sense, it is probable that the particles of matter 

 are absolutely impenetrable to each other. This impenetrability is not 

 however commonly called into effect in cases of apparent contact. If the 

 particles of matter constituting water, and steam, or any other gas, are of 

 the same nature, those of the gas cannot be in perfect contact ; and when 

 water is contracted by the effect of cold, or when two fluids have their 

 joint bulk diminished by mixture, as in the case of alcohol or sulfuric 

 acid, and water, the particles cannot have been in absolute contact before, 

 although they would have resisted with great force any attempt to com- 

 press them. Metals too, of all kinds, which have been melted, become 

 permanently more dense when they are hammered and laminated. A still 

 more striking and elegant illustration of the nature of repulsive force is 

 exhibited in the contact of two pieces of polished glass. The colours of 

 thin plates afford us, by comparison with the observations of Newton, the 

 most delicate micrometer that can be desired, for measuring any distances 

 less than the ten thousandth of an inch : it was remarked by Newton him- 

 self, that when two plates of glass are within about this distance of each 



