388 LECTURE XLI. 



of the matter that constitutes the earth, we come naturally to the properties 

 and powers of its individual particles, and to the phenomena of heat, 

 electricity, and magnetism, which are either qualities of matter, or depen- 

 dent on substances differing in some respects from common matter ; and 

 in the next place, to the combination of all these substances and actions in 

 meteorology, and in the phenomena of vegetable and animal life, a general 

 view of which will complete our discussions on the subject of physics. 

 The science of chemistry, or the doctrine of the qualities of particular 

 kinds of matter, might be said to belong to the investigation of the proper- 

 ties of matter in general ; but this science is of too great extent 1 ' and 

 importance to occupy a subordinate place in a system of natural philoso- 

 phy, and must, therefore, be considered as requiring a separate i^urse of 

 study. 



In our astronomical inquiries, we shall first examine the phenomena of 

 the heavens and earth in their simplest form, not as they immediately 

 appear to our observation, but as they are shown by unexceptionable 

 proofs to be naturally arranged. The stars and sun, the planets and their 

 satellites, and lastly the comets, will be severally described ; the causes of 

 the motions of these bodies will be superficially indicated ; their sensible 

 effects with respect to the inhabitants of the earth will be shown, and the 

 practical modes of determining their .situations and orbits will be ex- 

 plained. 



When we begin to consider, on a large scale, the affections of matter 

 and of space, we are impressed, at the first sight, with the inconceivable dis- 

 proportion between the magnitude of space and of sensible matter ; and we 

 are naturally led to inquire if the apparently void expanse of the universe 

 is wholly without all matter or all substance. The atmospheres of the planets 

 cannot indeed be said absolutely to terminate at any given point, but they 

 must become rare beyond all imagination at a very moderate distance. 

 The substance which produces the sensation of light must, however, be 

 every where found, at least without any sensible interval ; for if an eye 

 were placed in any point of the regions of unbounded space, wherever 

 human investigation or fancy can penetrate them, some luminous object 

 would at each instant be visible to it, and, in general, objects without 

 number might be seen in every direction. Light, therefore, must be every 

 where present, whether we suppose it to consist of separate projected cor- 

 puscles, or to be an affection of a highly elastic ether, pervading the uni- 

 verse in a state so rare, that although it constitutes a continuous medium, 

 it suffers all bodies to move through it without sensible resistance, and is 

 admitted even into their pores with perfect freedom ; and if we follow 

 Newton's opinion of the nature of light, we must suppose both such an 

 ethereal medium, nearly at rest, and the particles of light also, moving 

 swiftly through it, to exist together in all places ; to say nothing of the 

 possibility of the coexistence of a thousand other unseen and unknown 

 substances, essences, and influences, in the same individual place, which 

 may for ever set at defiance the pride of a presumptuous philosophy, 

 that would aspire to comprehend, within its own contracted sphere, the 

 whole extent of the mighty work of the creation. 



