ON THE ESSENTIAL PROPERTIES OF MATTER. ^465 



for the electrical fluid, if such a fluid exists, is probably attracted by mat- 

 ter, and yet it seems to be different in most respects from any modification 

 of common matter. A similar difficulty would occur if we attempted to 

 define matter by its impenetrability or mutual repulsion, or if we consi- 

 dered every thing as material that is capable of affecting the senses. We 

 must, therefore, take it for granted that matter is known without a defi- 

 nition, and we may describe it as a substance occupying space, or as a 

 gravitating or ponderable substance. 



It cannot be positively determined whether matter is originally of one 

 kind, owing its different appearances only to the form and arrangement of 

 its parts ; or whether there are various kinds of simple matter, essentially 

 distinct from each other ; but the probability appears to be in favour of 

 the former supposition. However this may be, the properties of matter 

 are by no means so simple in their nature, nor so easily reducible to gene- 

 ral laws, as the more mathematical doctrines of space and motion ; and 

 since our knowledge of them depends more on experience than on abstract 

 principles, they may properly be considered as belonging to particular 

 physics. We have found no inconvenience from the omission of the doc- 

 trine of matter as a part of the subject of mechanics ; although, in treating 

 of the strength of materials, as subservient to practical mechanics, it was 

 necessary to consider the effects of some of these properties as deduced from 

 experiment ; but it will appear that it was impossible to examine their 

 origin and mutual connexion, without supposing a previous knowledge of 

 many other departments of natural philosophy. 



We may distinguish the general properties of matter into two principal 

 classes, those which appear to be inseparable from its constitution, and those 

 which are only accidental, or which are not always attached to matter of 

 all kinds. The essential properties are chiefly extension and divisibility, 

 density, repulsion, or impenetrability, inertia, and gravitation ; the acci- 

 dental properties are in great measure dependent on cohesion, as liquidity, 

 solidity, symmetry of arrangement, cohesive elasticity, stiffness, toughness, 

 strength, and resilience. 



The extension of matter can scarcely be considered as a property sepa- 

 rate from its impenetrability, unless we conceive that it can occupy space, 

 without excluding other bodies from it. This opinion has indeed been 

 maintained by some philosophers, who have imagined that the minute 

 particles which they suppose to constitute light, may penetrate the ulti- 

 mate atoms of other matter without annihilating or displacing them ; 

 and if this hypothesis were admitted, it would be necessary to consider 

 each particle of matter as a sphere of repulsion, extended without being 

 impenetrable. 



The divisibility of matter is great beyond the power of imagination, but 

 we have no reason for asserting that it is infinite ; for the demonstrations 

 which have sometimes been adduced in favour of this opinion, are obvi- 

 ously applicable to space only. The infinite divisibility of space seems to 

 be essential to the conception that we have of its nature ; and it may be 

 strictly demonstrated, that it is mathematically possible to draw an infinite 

 number of circles between any given circle and its tangent, none of which 



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