phænomena depend upon one fluid in excess in the bodies positively electrified, and in deficiency in the bodies negatively electrified, or upon two different fluids capable by their combination of producing heat and light, or whether they may be particular exertions of the general attractive power of matter, it is, perhaps, impossible to decide, in the present imperfect state of our knowledge."*

And hence, heat, in the view of Sir Humphry Davy, Count Rumford, and various other justlycelebrated chemists and philosophers of the present day, coincidently with the doctrine of the Peripatetic school, is a mere property of matter, and not a substance sui generis, as was contended for by the Epicureans, in opposition to the disciples of Aristotle, and is contended for by the disciples of Boerhaave, Black, Crawford, and most of the chemists of our own times. The cause of heat, by those who deny it a substantive existence, is said to consist in a vibratory motion of the constituent particles of the heated body, too rapid to be traced by the eye. And as it is known to every one that bodies in general, as they become heated, occupy a larger space, and have their particles more widely repelled and separated from each other than in a colder temperature, it has of late become a favourite doctrine that the repulsive power, which in our last lecture we noticed to exist throughout matter, depends altogether upon the property of heat; in consequence of which Sir Humphry Davy uses heat and calorific repulsion as synonymous terms, and hence regards heat and gravitation, or general attraction, as anta gonist powers.

Elem. p. 176.

There is much plausible reasoning to be urged in favour of this hypothesis. It will as readily account for many, perhaps most, of the phænomena which accompany bodies in their change from one temperature to another, as the position of the substantive form of heat, and has some advantage in point of simplicity; but it is opposed by a variety of facts of so stubborn and intractable a nature, that no efforts of ingenuity have hitherto been capable of bending them into the service of the new doctrine. I observed, for instance, in our last lecture, that when two plates of glass are within a ten-thousandth part of an inch of each other, they cannot be made to approach nearer without a strong additional pressure. I observed, farther, that Professor Robison has calculated the extent of this pressure from actual experiment, and finds it amount to not less than a thousand pounds weight for every square inch of the glass. Now this resistance or repulsive power between the two plates of glass takes place equally under an air-pump and in the fullest exposure to the air of the atmosphere, while it appears to cease under water. By what cause the repulsion is excited in the two former instances, or disappears in the latter, we know not; but it does not seem possible for any ingenuity of argument to connect this repulsive power with heat, whether regarded as a substance or a mere property.

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 very considerably to lessen its dimensions; and it was on account

of this peculiar property that Mr. Wedgewood selected this last material for the purpose of forming his celebrated pyrometer, or instrument for measuring intense 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 proportion as additional heat is applied; but below 42° it begins to expand also upon exposure to cold, and continues to expand in nearly the same ratio upon the application of additional cold, till at 32° it freezes and becomes fixed. This curious phænomenon 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 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 quality 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 perpetually 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 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 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 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 originallly 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 diffused 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 nebulosity is, in such part, converted into a denser nucleus, which, by its comparative 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 nebulæ, or smaller nebulous clouds; and if some of them be much minuter than others, the minuter may at length attend upon the larger, as satellites upon a planet: and he (Herschel) gives instances of all these phænomena actually completed, as he conceives, or in a train of completion, in different parts of the visible heavens.

Such he submits as his latest opinion of the general construction of the heavens; believing stars, planets, and comets to have originated, and to be still originating, from such a source; the nebulous matter contained in a cubical space seen under an angle of ten degrees demanding a condensation of two trillion and two hundred and eight thousand billion times before it can be so concentrated as to constitute a globe of the diameter and density of

our sun.