ON THE ESSENTIAL PROPERTIES OF MATTER. 6ll 



life, wherever living beings can be placed, may therefore speculate with free- 

 dom on the possibility of independent 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 impene- 

 trability of matter inthis 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 na- 

 ture, those of the gas cannot be in perfect contact; and when water is con- 

 tracted by the effect of cold, or when two flukls have their joint bulk di- 

 minished by mixture, as in the case of alcohol, or sulfuric aciti, and water, 

 the particles cannot have been in absolute contact before, although they 

 would have resisted with great force any attempt to compress them. JNIetals 

 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* Th^ 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 himself, that when two plates of glass are within 

 about this distance of each other, or somewhat nearer, they support each 

 other's weight in the same manner as if they were in actual contact, and 

 that some additional force is required, in order to make them approach still 

 nearer ; nor does it appear probable that th^; contact is ever perfect, other- 

 wise they might be expected to cohere in such a manner as to become one 

 mass. Professor Robison has ascertained by experiment the force necessary 

 to produce the greatest possible degree of contact, and finds it equivalent to 

 a pressure of about a thousand pounds for every square inch of glass. It is 

 therefore obvioua that in all common cases of the contact of two distinct 

 bodies, it n)ust be this repulsive force that retains them in their situation. 

 I have found that glass,placed on a surface of metal, exhibits this force nearly 

 in the same degree as if placed on another piece of glass ; it is also inde- 

 pendent of the presence of air; but under water, it disappears.. 



