ESSENTIAL AND PECULIAR. 



53 



becomes fixt. This curious phaenomenon has never been accounted for« 

 If calorific repulsion produce the expansion above 42^, what is it that pro- 

 duces the same etfect below ? We can, perhaps, explain the cause of the 

 expansion during the act of freezing, from the peculiar shape of the crystals 

 which the water assumes in the act of consolidating ; but this explanation 

 will in no respect apply to the expansion of the water when it reaches the 

 freezing point. In this curious and unillustrated fact cold appears to be 

 as much entitled to the character of a repulsive power as heat. 



For these and numerous other reasons, therefore, heat is even at the 

 present moment usually regarded, not as a mere quahty of body produced 

 by internal vibration, and forming an antagonist power to the attraction of 

 cohesion, but as a distinct and independent substance. The sources of 

 heat are various, though by far the principal reservoir throughout the whole 

 solar system is the sun himself, which Dr. Herschel believes to be perpetu- 

 ally secreting the matter of heat from those dark and discoloured parts on 

 its surface which we call spots, by many astronomers regarded as volcanoes, 

 and many of which are larger than, and some of them five or six times as 

 large as, the diameter of the earth ! This material Dr. Herschel supposes 

 to be first thrown off in the form of an atmosphere, and afterwards this 

 atmosphere to be diflfused in every direction through the whole range of 

 the solar empire : and, in the Philosophical Transactions for 1 801, he has 

 endeavoured to show that the variation in the heat of diflferent years is 

 owing to the more or less copious supply of fuel which such spots com- 

 municate. 



This opinion I at present merely glance at ; as it is my intention on a 

 future occasion to examine its validity, as well as to trace out the other 

 sources from which heat is derived, and to take a survey of the laws by 

 which it is regulated. It will form a progressive part of that investigation 

 to follow up the general nature of light ; to try the question whether it be a 

 substance or a property ; and if a substance, whether distinct from a mere 

 modification of heat. I shall at present only observe, that, in one of the 

 latest opinions of the philosopher to whom I have just adverted, it is not 

 only a substance, but the source of all visible substances, and the basis of 

 all worlds. 



Dr. Herschel has recently taken great pains to prove, but with no small 

 degree of repugnancy to a former hypothesis of his, that the luminous fluid 

 which so often appears in the heavens on a bright night, and shoots streaks 

 athwart them, is diflfused light, existing independently of suns or stars, 

 though perhaps originally thrown forth from them ; another kind of ethereal 

 matter being sometimes united with that of light, and hence rendering it at 

 times capable of opacity. In this diflfused state he calls every distinct mass 

 a nebulosity ; he conceives all its particles to be subject to the common 

 laws of gravitation, or the centripetal force ; and that certain circumstances, 

 unknown to us, may have occasionally produced a nearer approximation 

 between some particles than between others ; whence the diffused nebu- 

 losity is, in such part, converted into a denser nucleus, which, by its com- 

 parative preponderancy, must lay a foundation for a rotatory motion, and 

 attract and determine the circumjacent matter still more closely to itself, 

 and consequently diminish the extent of the nebulous range. 



The nuclei thus arising may sometimes be double or triple, or still more 

 complicated ; and whenever this occurs, the nebulosity will be broken into 

 different nebulae, or smaller nebulous clouds ; and if some of them be much 

 sninuter than others, the minuter may at length attend upon the larger, 



