64 ON THE PROPERTIES OF MATTER, 



of 1680 within a sixth of its diameter, or as near as 147,000 miles, its tail be- 

 ing- of a like length. 



There is one comet, however, that we seem to be somewhat better ac- 

 quainted with than with this that paid us so near a visit, or indeed than with 

 any other, from its having approached us visibly for four times in succession, 

 if not oftener. It was towards the beginning of last century that Mr. Halley 

 was struck with the remark, that the general elements and character of the 

 comets observed in 1531, 1607, and 1682, were nearly the same ; whence he 

 concluded that the whole formed but one identical body, that took about 

 seventy-six years to complete its eccentric orbit ; and hence, although in 

 consequence of this eccentricity, and its travelling amid a range of heavenly 

 bodies that are altogether invisible to us, and whose influence seems to bid 

 defiance to calculation, it is difficult to form an estimate of its progress, he 

 ventured to suggest, that it would appear again, making due allowances for 

 these incidents, towards the close of 1758, or the commencement of 1759 : 

 and he had the high satisfaction of seeing his prediction verified ; the comet 

 passing- its perihelion March 12th, 1759, within the limits of the errors of 

 xvhich he thought his results susceptible. It is apparently this comet, which 

 at this last period only excited the curiosity of astronomers and mathemati- 

 cians, that in 1456, or four revolutions earlier, towards the close of what are 

 called the dark ages, spread such consternation over all Europe, already, 

 indeed, terrified by the rapid successes of the Turkish arms, that Pope Cal- 

 lixtus was induced to compose a prayer for the whole western church, in 

 which both the Turks and the comet were included in one sweeping- ana- 

 thema. 



Admitting the truth of Dr. Herschel's hypothesis, as we are now contem- 

 . plating 1 it, it is possible that some of the lately discovered planets, which are 

 now attendant upon the sun, were formerly comets, whose orbits have for 

 ages been growing progressively more regular, as well as their constitutional 

 rudiments more dense ; and such, indeed, is the opinion of M. Voigt, and of 

 various other philosophers on the continent. 



The object of the present and the preceding lecture has been to submit a 

 sketch of the most obvious properties belonging to MATTER, so as to enable 

 you to obtain a bird's-eye view of the general phenomena it is capable of as- 

 suming, and the general changes it is necessarily sustaining. From the qua- 

 lities I have placed before you, of passivity, cbhesibility, divisibility, and at- 

 tractions of various kinds, must necessarily result, according to the intensity 

 with which they are called into action, the phenomena of liquidity, viscidity, 

 toughness, elasticity, symmetry of arrangement, solidity, strength, and resi- 

 lience. But the powers which thus perpetually build up the inorganic world, 

 and to this our survey has been entirely confined, perpetually also destroy 

 it : for. the whole, as I have had occasion to observe, is a continued circle of 

 action ; a circle most wise, most harmonious, most benevolent : and hence 

 as one compound substance decays, another springs up in its place, and can 

 only spring up in consequence of such decay. 



There is, however, another lesson, if I mistake not, which we may readily 

 learn from these lectures, however imperfectly delivered, and which is alto- 

 gether of a moral character : I mean that of humility, in regard to our own 

 opinions and attainments ; and of complacency, in regard to those of others. 

 After a revolution of six thousand years, during the whole of which period of 

 time the restless ingenuity of man has been incessantly hunting- in pursuit of 

 knowledge, what is there in physical philosophy that is thoroughly and per 

 fectly known even at the present moment 1 and of the little that is thus 

 known, what is there which has been acquired without the clash of contro- 

 versy and the warfare of opposing speculations 1 Truth, indeed, for ever 

 praised be the great Source of Truth, for so eternal and immutable a decree 

 has at all times issued, and at all times will issue, from the conflict ; but 

 while we behold philosophers of the highest reputation, philosophers equally 

 balanced in the endowment of native genius, proved by the great teacher 

 Time to have been alternately mistaken upon points to which they had hen 



