Fahrenheit, which is within a few degrees of the boiling point of water, was borne by the late Dr. Fordyce, during ten minutes.* And it is highly worthy of notice, as connected with the general intention of this Treatise, that, during the same time, a thermometer which had been fixed under his tongue indicated only the 98th degree of Fahrenheit: so that the body remained very nearly of its natural temperature, during its exposure to an atmosphere exceeding its own temperature by full 100 degrees.†

This uniformity of animal temperature, under such circumstances, is in a great measure owing to the process of evaporation, which takes place from the general surface of the body, and from the air-vessels of the lungs for if animals are confined in a chamber, the atmosphere of which is so moist, that no evaporation can take place from the surface of their bodies, it has been found that their temperature is as capable of being steadily and uniformly raised, by increasing the heat of the room in which they are placed, as if they were inanimate matter.

of

The application of heat to the various purposes life has a very extensive range; and with reference to the daily preparation of the more common forms of our food, whether animal or vegetable, distinguishes the habits of man from those of every other species. Without the power indeed of commanding the application of heat in its various degrees, many of the most important arts of civilized society would fail.

Without that power, how could clay be hardened into the state of brick, of which material most of the habitations in many large cities are constructed? Without the aid of the same agent, how could quicklime, the base of every common cement, be produced from limestone? without the application of the higher Phil. Trans. 1775. vol. lxv. p. 117. † Ibid. p. 118. For an account of similar experiments carried to a further extent, see p. 484, &c. of the same volume of the Phil. Trans.

degrees of heat, metals could neither be reduced from their ores, nor the reduced metals worked into convenient forms. Neither, without the same aid, could that most useful substance glass be produced; a material, which, in comparison hardly known to the ancients, has in modern times become almost indispensably necessary to persons of the poorest class, as a substance of daily use for various economical purposes. But if we consider the properties of this valuable compound, with reference to the aid derived from it in the investigations of science, there are few substances of higher importance to the philosopher. Among the most useful of those properties are its impermeability to fluids, either in a liquid or aeriform state; its ready permeability to light, together with its power of modifying the qualities of that fiuid; and its resistance to almost all those chemical agents, which are capable of destroying the texture of most other substances with which they remain long in contact.

In considering the extensive utility of the thermometer and barometer, in their common and most convenient forms, it is evident that their practical value almost entirely depends on the transparency of glass, and on its impermeability to air: for if the glass, of which they are made, were opaque, the variations in the level of the quicksilver contained within them would be imperceptible to the eye; and could not be indirectly ascertained, unless by very circuitous and difficult means: and, on the other hand, if the glass were permeable to air, the variation in the level of the quicksilver, in the case of the barometer at least, would necessarily be prevented. The same properties of transparency and impermeability to air very greatly enhance, if they do not solely constitute the value of glass, in all those philosophical experiments which are carried on under what is called the exhausted receiver.

But the most important result of the transparency of glass is the modification which light undergoes in

its passage through lenticular masses of that material. When, for instance, in consequence of disease or advancing age, the eye no longer retains the power of discerning objects distinctly, how much of hourly comfort, as well as of intellectual enjoyment, would be lost, were we not able to supply the natural defect by the artificial aid of glasses of the requisite form and density. And, again, how many important facts in the physiology of animals and vegetables, as also in the constitution of inanimate bodies, would have remained for ever undiscovered, but for the aid of the microscope; the magnifying powers of which depend on the transparency, and form, and the right adjustment of those pieces of glass through which the objects subjected to observation are viewed?

And, lastly, how shall we estimate the value of those discoveries, to say nothing of the constantly accumulating mass of observations connected with them, which the world owes to that wonderful instrument the telescope? By the aid of which not only has the knowledge of our own sidereal system been extended, in consequence of the discovery of new planets belonging to it; but it seems to have been rendered highly probable that those obscurely defined luminous masses, which sir William Herschel termed nebula, observable within the limits of individual constellations, are really the accumulated light of innumerable stars seen through the medium of a space hitherto immeasurable: and that the milky way itself is an extended accumulation of similar nebulæ; the collected light of which, at some inconceivable point of distance, may appear to the inhabitants of still more distant spheres, as a mere speck. Dare the mind attempt to penetrate beyond this general statement, and to speculate upon the characters of its detail? What if there be a resemblance, or even an analogy, between the structure and inhabitants of this earth and of the other planets of our system? What if every fixed star which we either see with

the naked eye or by the aid of the telescope, or whose existence we can conceive on probable grounds by the mind's eye, be itself the centre of a system consisting, like our own, of numerous subordinate spheres, and every one of these inhabited by responsible agents, like ourselves; to whose uses both inorganic elements and animals and vegetables, analogous if not similar to our own, may be subservient? What if the moral history and state of the inhabitants of those numberless spheres be like that of man?—But the view, which the investigation of this question seems capable of unfolding, is too awful for the eye of reason; and, however its discussion might magnify our conviction of the infinite power and goodness of the Creator, is not to be approached perhaps without culpable presumption.

Let us therefore return to considerations more appropriate to the character of human knowledge: and, having referred to the effects produced by heat on various forms of matter, let us inquire what facilities nature has placed within our reach for the purpose of exciting and maintaining heat itself. The chemist in his laboratory, surrounded by the numerous and various agents which he is constantly employing, can never have any difficulty in producing the vestal element. By concentration of the sun's rays he may inflame any combustible substance: by compression of common air in a small cylinder of glass, or metal, he may ignite a piece of fungus, or inflame a piece of phosphorus, attached to the extremity of the piston which is employed to compress the air. He may instantaneously produce flame by pouring concentrated nitric acid on oil of turpentine, or on certain saline compounds; by the simple trituration of phosphorus, or other chemical agents; by directing a small stream of inflammable air on minute particles of platina loosely aggregated in a state somewhat resembling sponge; or, not to accumulate too many instances, he may delight himself for the thousandth time by ig

niting a fine wire of steel, in passing the electric current along it by means of the Voltaic apparatus.*

There are few individuals however who have commonly such magic instruments at hand: and, even if they had, it is probable that they would want both the leisure and inclination to preserve them in a state fitted to produce at any moment the intended effect; for, though each successive year has of late given birth to some new form of apparatus calculated to produce instantaneous light, we find ourselves constantly recurring to the flint and steel, which our forefathers of many generations have used; and which will doubtless be the staple apparatus of our latest posterity.

The more important part of the present inquiry remains to be considered, the means namely of maintaining heat, when once excited, to a sufficient extent and degree of intensity for the various purposes of

* It will not perhaps be deemed impertinent, to relate an instance of the sagacity of the late. Dr. Wollaston, in connexion with the present subject. It happened to the author of this Treatise, at a comparatively early period of his life, to deliver a letter of introduction to Dr. Wollaston at a moment when that philosopher was engaged in conducting an electric current, by means of the Voltaic apparatus, through three portions of fine steel wire, differing from each other in diameter. With that vivacity of manner, which in him resulted rather from the simple consciousness of the acquisition of truth, than from the ignoble triumph of individual superiority, he asked which of those wires would first become of a red heat; and being answered, at a hazard rather than from any reasonable ground of conjecture, that a red heat would perhaps first take place in the thickest of the three "I expect it will," he said, "and that the finest wire will never reach a red heat; for I conclude that, from its extreme fineness, the heat excited in it will be dissipated by radiation so rapidly, as to prevent the accumulation of a quantity sufficient for its ignition." It need hardly be added that the conjecture was verified.

As an instance of the minute scale on which Dr. Wollaston was in the habit of carrying on his philosophical investigations, it may be mentioned that the preceding experiment was conducted in a single cell of a single and moderately sized Voltaic trough.

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