62 ON THE PROPERTIES OF MATTER, 



Heat, again, which undoubtedly makes the particles of iron repel each other, 

 so that given weights of them occupy a larger space makes the particles of a 

 ball of clay, on the contrary, attract each other into a closer approximation, so 

 as veiy considerably to lessen its dimensions ; and it was on account of this 

 peculiar property that Mr. Wedgewood selected this last material for the pur 

 pose of forming his celebrated pyrometer, or instrument for measuring in- 

 tense heats, the increase of the heat being indicated by the decrease of the 

 mass of clay. 



So water at about 42 of Fahrenheit, which forms its medium of density, 

 begins to expand upon exposure to heat, and continues to expand in propor- 

 tion as additional heat is applied ; but below 42 it begins to expand also 

 upon exposure to cold, and continues to expand in the very same ratio upon the 

 application of additional cold, till at 32 it freezes and becomes fixed. This 

 curious phenomenon has never been accounted for. If calorific repulsion 

 produce the expansion above 42, what is it that produces the same effect 

 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 curi- 

 ous 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 pre- 

 sent moment usually regarded, not as a mere quality of body produced by 

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

 sion, 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 perpetually 

 secreting the matter of heat from tttose dark and discoloured parts on its sur- 

 face 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 afterward this atmosphere to 

 be diffused in every direction through the whole range of the solar empire : 

 and, in the Philosophical Transactions for 1801, he has endeavoured to show 

 that the variation in the heat of different years is owing to the more or less 

 copious supply of fuel which such spots communicate. 



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

 ture 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 

 property ; and if a substance, whether distinct from or 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 diffused 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 li^ht, and hence rendering it at times 

 capable of opacity. In this diffused ;,t.alj lie calls every distinct mass a ne- 

 bulosity ; he conceives all its particles to be subject to the common laws of 

 gravitation, orlhe 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 nebulosity is, in such 

 part, converted into a denser nucleus, which by its comparative preponde- 

 rancy, must lay a foundation for a rotatory motion, and attract and deter- 

 mine the circumjacent matter still more closely to itself, and consequently, 

 diminish the extent of the nebulous range. 



