606 ^ LECTURE XI.IX. 



thickness, as to be capable of destroying a portion of violet light under the 

 same circumstances. 



We are at present to descend from the affections of the large masses of matter, 

 which fonn the great features of the universe, to the particular properties of 

 the matter which constitutes them, as far as they are common to all matter 

 in general; but those properties which are peculiar to certain kinds of matter 

 only, being the subjects of chemical science, are not to be included In the 

 discussion. If we are asked for a definition of matter, it will be somewhat 

 difficult to avoid all circuitous expressions. We may make gravitation a test of 

 matter, but then we must say, that whatever is attracted by other matter, is 

 also to be denominated matter, and this supposes the subject of our definition 

 already known; besides that tiie property of attraction may also possibly 

 belong to substances not simply material ; for the electrical fluid, if such a 

 fluid exists, is probably attracted by matter, and yet it seems to be different 

 in most respects from any modification of common matter. A similar diffi- 

 culty would occur if we attempted to define matter by its impenetrability or 

 mutual repulsion, or if we considered every thing as material that is capable 

 of aftecting the senses. We must, therefore, take it for granted that matter 

 is known without a definition, and we may describe it as a substance occupy- 

 ing 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 general laws, as 

 the more mathematical doctrines of space and motion ; and since our know- 

 ledge 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 doctrine of matter as apart of 

 the subject of mechanics ; although, in treating of the streugth of materials, 

 as subservient to practical mechanics, it was necessary to consider the effit^cts 

 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 



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