is a name given to the space in which a thermometer may be placed, so that it shall be affected by the sun's rays refracted by a prism. It is, in part, the same with the Prismatic Spectrum, which exhibits the different colours produced by the solar light.

The philosophical instrument now called a thermometer, was first named thermoscope; and was prized by the naturalists, because it gave him indications of the presence and agency of fire in many cases where our sensation of warmth or heat was unable to discover it. It was not long before it was observed that it also affords us measures of the changes which take place either in the quantity or the activity of the cause of heat, and of many other important phenomena usually accompanied by heat. They were then called thermometers. But in both of these offices, it is still a doubt whether it indicates and measures any real substance, a being sui generis, to which we may give the name fire, phlogiston, caloric, heat, or any other; or only indicates and measures certain states or conditions, in which all bodies may be found, without the addition or abstraction of any material substance.

We think that this question has a greater chance now of being decided than in any former time, in consequence of a recent and very important discovery made by that unwearyed observer of the works of God, the celebrated Dr Herschel. Being greatly incensed when looking at the sun, by the great heats produced in the eye-pieces of his telescopes, he thought that the laws of refraction enabled him to diminish them by a proper construction of his eye-pieces. He began his attempts like a philosopher, by examining the heat produced in the various parts of the prismatic spectrum. Comparing the gradation of heat with that of illumination, he found that they did not, by any means, follow the same law. The illumination increased gradually from the violet end of the spectrum, where it was exceedingly faint, to the boundary of the green and yellow, where it was the most remarkable; and after this, it decreased as the illuminated object approached the red extremity of the spectrum. But the calorific power of the refracted light increased all the way from the extreme violet to the extreme red; and its last augmentations were considerable, and therefore unlike the usual approaches of a quantity to its maximum state. This made him think of placing the thermometer a little way beyond the extremity of the visible spectrum. To his great astonishment, he found that the thermometer was more affected there than in the hottest part of the illuminated spectrum. Exposing the thermometer at various distances beyond the extreme red, but in the plane of refraction, he found that it was most strongly affected when placed beyond that extremity, about one-fifth of the whole length of the spectrum; from thence the calorific influence of the sun gradually diminished, but was still very considerable, at a distance from the extreme red equal to three-fifths of the length of the luminous spectrum. These first-suggested modes of trial appeared to Dr Herschel to be too rude to intitle him to say that the warming influence did not extend still farther. Indeed the instrument scarcely performed the part of a thermometer, but merely that of an indicator of heat, or a thermoscope.

Here is a very new, and wonderful, and important piece of information. We apprehend that all the philosophers of Europe, as well as the unlearned of all nations, believe that the warming influence of the sun, and of other luminous bodies, is conjoined with their power of illumination. Most of the philosophers admitted the emission of a matter called light, projected from the shining body, and moving with astonishing velocity, in those lines which the mathematicians called rays, because they diverged from the shining point, as the radii or spokes of a wheel diverge from the nave. This notion seems to be the simple suggestion of Nature; and it also seems to be the opinion entertained by Sir Isaac Newton. His demonstration of the laws of reflection and refraction proceeds on this supposition alone, and the particles of light are held by him to be affected by accelerating and deflecting forces, in the same way as a stone thrown from the hand is affected by gravity. Huyghens, indeed, Dr Hooke, and Euler, imagined that vision and illumination were effected in the same way that hearing, and resonance, and echo, are effected—that there is no matter projected from the shining body; but that we are surrounded by an elastic fluid, which is thrown into vibrations by certain tremors of the visible object—and that those vibrations of this fluid affect our eye in the same way as the undulation of elastic air, produced by the tremors of a string or a bell, affect our ear. According to these philosophers, a ray of vision is merely the line which passes through all these undulations at right angles.

These two opinions still divide the mathematical philosophers of Europe; but the majority, and particularly the most eminent for mathematical and mechanical science, are (with the exception of Huyghens and Euler) on the side of the vulgar. This opinion has been greatly strengthened of late years by the discoveries in chemistry. The influence of light on the growth of plants, the total want of aromatic oils in such as grow in the dark, and their formation and appearance in the very same plant, along with the green colour, as soon as the plant is placed in the light (even that of open day without sunshine, or in the light of a candle), is a strong indication of some substance being obtained from the light, absorbed by the plant, and combined with its other ingredients. The same conclusion is drawn from the the effects of the sun's light on vegetable colours, on the nitric and nitrous acids, on manganese, on the calces or oxyds of metals, and numberless other instances, which all concur in rendering it almost unquestionable that the sun's rays, and those of other shining bodies, may be, and daily are, combined with the other substances of which bodies are composed, and may be again separated from them. And, should any doubts remain, it would seem that the theory of combustion, first conceived and imperfectly published by Dr Hooke in his Micrography, p. 103, and in his Lampas, p. 1, &c., adopted by Mayow (see Hooke and Mayow in this Suppl.), forgotten, and lately revived and confirmed by Mr Lavoisier, removes them entirely. In the beautiful and well-contrived experiments of the last gentleman, the light, accompanied by its heat, which had been absorbed in the process of growth or other natural operations, reappeared in their primitive form, and might again be absorbed and made to undergo the same round of changes.

Scheele, not inferior to Newton in caution, patience, and accuracy, and attentive to every thing that occurred in his experiments, discovered the separability of the illuminating and the warming influences of shining bodies. He remarked, that a plate of glass, the most colourless and pellicoid that can be procured, when suddenly interposed between a glowing fire and the face, instantly cuts off the warming power of the fire, without causing any sensible diminution of its brilliancy. He followed this discovery into many obvious consequences, and found them all fully confirmed by observation and experiment. The writer of this article, immediately on hearing of Scheele's experiments, repeated them with complete success; but he found, that when the glass plate had acquired the highest temperature which it could acquire in that situation, it did not any longer intercept the heat, or at least in a very small and almost insensible degree. It seemed to absorb the heat, till saturated, without absorbing any considerable portion of the light.

This separability of heat from light does not seem to have met with the attention it deserved. Dr Scheele's untenable theories on these subjects turned away the attention of the chemists from this discovery, and the mathematical philosophers seem not to have heard of it at all. The late Dr Hutton of Edinburgh was more sensible of its importance; and in his last endeavours to support the falling cause of phlogiston, makes frequent allusions to it. But in his attempts to explain the curious observations of Messrs Sandfure and Pictet, in which there are unquestionable appearances of radiated heat, he reasons so unconsequentially, that few readers proceed farther, so as to notice several observations of facts where the illuminating and warming influences are plainly separated. In all these instances, however, Dr Hutton considers the invisible rays as light, but not as heat; maintaining that they are invisible, or do not render bodies visible, only because our eyes are insensible to their feeble action.

It was reserved for Dr Herschel to put this matter beyond dispute by these valuable experiments. For did the invisibility of any of the light beyond the extreme red of the prismatic spectrum arise from the insensibility of our organs, the spectrum would gradually fade away beyond the red; but it ceases abruptly. These thoughts could not escape this attentive observer. He therefore examined more particularly those invisible rays, causing them to be reflected by mirrors, and refracted through lenses; and, in short, he subjected them to all the subsequent treatments which Newton applied to the colouring rays. He found them retain their specific refrangibilities and reflectivities with as much uniformity and obstinacy as Newton had observed in the colour-making rays. They were made to pass through lenses while the illuminating rays were intercepted by an opaque body, and the invisible rays were then collected into a focus. They were reflected, both by the anterior and posterior surfaces of transparent bodies. In all these trials they retained their power of expanding the liquor of a thermometer, and exciting the sensation of heat.

These trials were not confined to the solar light or the solar rays: They were also made on the emanations from a candle, from an open fire, and from red hot iron; then they were made with bodies not hot enough to shine; with the heat of a common stove, and the heat from iron which was not visible in the dark. The event was the same in all; and it was clearly proved that heat, or the cause of heat, is as susceptible of radiation as light is; and that this radiation is performed in both according to the same laws.

We look with impatience for the subsequent experiments of this celebrated philosopher on this subject; for we consider them as of the greatest and most extensive importance for explaining the operations of Nature. We see, with indubitable evidence, that there are rays from the sun, and other bodies, which do not illuminate. It does not follow, however, that there are rays which do not warm; for the thermometer was affected in every part of the coloured spectrum. Dr Herschel seems to think that the power of affecting the organ of sight depends on the particular degree of mechanical momentum which are indicated by the different degrees of refrangibility. We confess that we think it unlikely that such a power should terminate abruptly. We do not observe this in analogous phenomena: the evanescence of our sensations of sound, of musical pitch, of heat, &c. are all gradual. We think it more likely that illuminating and warming are specific effects of different things. We should have entertained this opinion independent of all other experience; and we think it strongly confirmed by the experiments of Dr Scheele already mentioned. We are disposed therefore to believe that there are rays which illuminate, but which do not warm; and rays which warm without illuminating. We have experiments in prospect, by which we hope to put this to the test.

These experiments of Dr Herschel afford another good argument for the common opinion concerning light, namely, that it is a matter emitted from the shining body, and not merely the undulations of an elastic medium; for if it were undulation, then, since there is heat in the yellow light, it would follow that a certain frequency of undulation produces both the sensation of heat and the sensation of a yellow colour. In this case they should be inseparable.

This follows, in the strictest manner, from the principles or assumptions adopted by Euler in his mechanical theory of undulations. The chromatic differences in the rays of light are affirmed to arise entirely from the different frequencies of the ethereal undulations; and he endeavours to show that these differences in frequency produce a difference in refrangibility. It is evident that this reasoning is equally conclusive with respect to the calorific or heating power of the rays. The light and the heat are both undulations; they differ only in frequency; and this frequency is indicated (according to Euler) by the refrangibility. There is a certain frequency therefore which excites the sensation of yellow. The same frequency, indicated by the same refrangibility, produces heat; therefore the frequency which produces this degree of heat also produces the sensation of yellow. We must not say that the momentum of the undulation may produce heat, but is insufficient for the production of light, as a string may vibrate too feebly for being heard; for we see, by Dr Herschel's experiments, that, with a momentum sufficient for making the most brilliant spectrum, there are rays (and those which have the greatest momentum) which produce heat, and yet are invisible.

It does not follow, from any of Dr Herschel's experiments, that the rays emitted by iron, which is not hot enough to shine in a dark room, have all the different degrees of refrangibility observed by him. Perhaps none of them would fall on the chromatic spectrum. We think, however, that this is not probable. It may be tried by collecting them to a focus by a lens, intercepting, however, all those which are less refrangible than the red making rays. We trust that the thermometer in the focus will still be affected.

This is but a very imperfect account of this important discovery; but we thought that it would be highly interesting to our readers. The press was employed on this very sheet when we received the information from a friend, who had seen Dr Herschel's Dissertation, which will appear in the first volume published by the Royal Society. We trust that the ingenious author will soon follow it up with the investigation of the subject in all its consequences.

We hope that he will examine what will result from mixing some of the invisible rays with some of the coloured ones. We know that the yellow and the blue, when mixed, produce the sensation of green. Perhaps the invisible rays may also change the appearance. We do not, however, expect this.

We also hope that Dr Herschel will examine whether the invisible rays of the sun produce any effect on vegetable colours; whether they blacken the calces of silver and bismuth, luna cornua, and decompose the nitrous and the oxygenated muriatic acid, &c. &c. We should thus get more insight into the nature of calorific and of combustion. Combustion may perhaps be restored to its rank in the phenomena of Nature, and no longer be sunk in the general gulf of oxygenation, and thus obliterated from the memory of chemists. It is perhaps the most remarkable phenomenon of material Nature; and fire and burning will never go out of the language of plain men. Fire, and all its concomitants, have, in all times, been considered as even the chief objects of chemical attention; and an unlearned person will state, when a chemist tells him that there is no such thing, and that what he calls the burning of a piece of coal is only the making it four. He will perhaps smile; but it will not be a smile of affect.

It was one darling object of the Revolutionary Committee of Chemists, assembled at Paris in 1787, to banish from our minds, by means of a new language, all remembrance of any thing which we did not derive from the philosophers of France. We think ourselves in a condition to prove this by letters to this country from the scene of action; in which the expected victory is spoken of in terms of exultation, and with so little restraint, that the writer forgets that it is Dr Black whom he is informing that l'air fixe and la pouvure philosophique will soon be forgotten; and yet the writer was a gentleman of uncommon modesty and worth, and sincerely attached to Dr Black. We give this as a remarkable instance of the esprit de corps, and of the nature and towering ambition of that nation. From this they have not swerved; and they hope to gain this summit of scientific dominion in the same way as the same philosophers hope to banish Christianity by means of their new calendar. It may, however, turn out that both Dr Hooke and Mr Lavoisier are mistaken, when they make the oxygen gas the sole source of both the light and the heat which accompany combustion. One of them may perhaps be furnished by the body which all, except the new philosophers, call combustible.

The objections which may be made to the theory of Huyghens and Euler, on the acknowledged principles of mechanics, appear to us unanswerable. Euler has never attempted to answer those taken from the different differing powers of different substances. The objections made to the Newtonian, or vulgar theory of emission, are not such as imply absurdity; they are only difficulties. The chief of them, viz., the fumens of velocity in all lights whatever, is of this kind. It is merely an improbability. But the objections to the theory of undulation, deduced from the chemical effects of light, are not less strong than those deduced from mechanical principles. It is quite inconceivable that the undulation of a medium, which pervades all bodies, shall produce aromatic oils in some, a green fecula in others, shall change sulphuric acid into sulphur, &c. &c. No effects are produced by the undulations of air, or the tremors of elastic bodies, which have the most distant analogy or resemblance to these.

That the sun and other shining bodies emit the matter of light and heat, seems therefore to merit the general reception which it meets with from the philosophers. But even of this class there are differences in opinion. Some imagine that light only is emitted, and that the heat which we feel is occasioned by the action of the luminous rays on our atmosphere, or on the ground. Were the sun's calorific rays as dense at the surface of the sun as his luminous rays are, the heat there must exceed (say they) all that we can form any conception of. Yet we see, that when the nucleus of the sun is laid bare by some natural operation, which, like a volcanic explosion, throws aside the luminous ocean which covers it to a prodigious depth, the naked parts of this nucleus are black. Therefore the intense heat in that place is not able to make it shining hot, as it does in all our experiments with intense heats, giving a dazzling glare. This is thought highly improbable; and it is therefore supposed that there it, primitively, no heat in the sun's rays, but that they act on our air, or other terrestrial matter, combining with it, and disengaging heat from it, or producing that particular state and condition which we call heat.

We think that Dr Herschel's discovery militates strongly and irresistibly against this opinion; and shows, that whatever reason we have for saying that the sun's rays bring light from the sun we have the same authority for saying that they bring heat, fire, caloric, phlogiston, or by whatever other name we choose to distinguish the cause of warmth, expansion, liquefaction, chalination, &c.

We must either say that light and heat are not substances of a peculiar kind, susceptible of union with the other ingredients of bodies, but merely a state of undulation of an elastic medium, as sound is the undulation of air; or we must say that the sun's rays contain light and heat, in a detached state, fit for appearing in their simplest form, producing illumination and expansion, and for uniting chemically with other matter. Whichever of these opinions we adopt, it is pretty clear that all attempts to discover a difference in the weight of hot and cold bodies may be given over. In the first case, it is self-evident; in the second, we have abundant evidence, that if light and heat, being gravitating matter, like all other bodies, were added to, or subtracted from bodies, in sufficient quantity to be sensibly heavy, the rays of the sun, or even the light of a candle, would occasion instant destruction by its mere momentum; since every particle of radiated light and heat moves at the rate of 200,000 miles in a second.

This discovery of Dr Herschel's adds greatly to the probability of the opinion which we expressed on another occasion, that the forces or powers of natural substances, which are the immediate causes of the chemical phenomena, are no way different from the mechanical forces which render bodies heavy, coherent, elastic, expansive, &c.; in short, that they are what we call accelerating forces. We deduced this from the fact, that mechanical force can be opposed to them, so as to prevent their action in circumstances where it would otherwise certainly take place. Thus, by external pressure, we can prevent that union of water and caloric which would convert it into elastic steam. We can even dilute them again, when steam is already produced, by forcibly condensing it into a smaller space. Now, the refraction and reflection of heat are performed according to the same precise laws which we observe in the refraction and reflection of light; and Sir Isaac Newton has demonstrated that those phenomena arise from the action of accelerating forces, whose direction is perpendicular to the acting surfaces. The matter of heat, therefore, is like other matter in its mechanical properties; and, in the motion of refraction, it is acted on and deflected, just as a projectile is acted on and deflected by gravity. It continues in motion till its velocity and direction are changed by deflecting forces, exerted by the particles of the transparent medium or the reflecting surface. It would take up too much room, but it is a very easy process, to demonstrate that this regular refraction of heat is altogether incompatible with the usually supposed notion of caloric; namely, that it is an expansive fluid like air, but incomparably more elastic: from which property very plausible explanations have been given of the elasticity of gases, steam, and such like fluids. Every intelligent mechanician will be sensible, that all this sort of chemical science falls to the ground, when it is proved, by exhibition of the fact, that radiated heat is refracted in the same way with radiated light. We must look for the explanation of the immense explosive force of fulminating silver, gold, &c., in some very different principles from those which are now in vogue. We apprehend, too, that the very phenomenon of this refraction gives indication of forces which are sufficiently powerful for this explanation: For when we reflect on the astonishing velocity of the ray of heat; on the minute space along which it is deflected, and consequently the time of this action, minute beyond all imagination; and when we compare these circumstances with the deflection produced by gravity in the motion of a projectile—it is evident that the deflecting force of refraction must exceed the greatest force that we have any knowledge of, in a greater proportion than the weight of Mount Aetna exceeds that of a particle of sand. We would desire Mr de la Place to suspend his hopes of establishing universal fatalism, till he can reconcile these phenomena with his fundamental principle, "that all forces which are diffused from a single point, necessarily and essentially diminish in the inverse duplicate ratio of the distances." Till he can do this, he had better allow, with Newton, that the selection of the duplicate ratio for the action of gravity (by which alone the solar system can be rendered permanent and orderly) is a mark of wisdom and benevolence. We would advise him to reconcile his mind to this; and perhaps, like the modest and admiring Newton, he may, in good time, find comfort in the thought.

It is also highly worthy of remark, that this refracting force, almost immeasurable, which is so plainly exerted between the particles of bodies and light, when considered as of the same kind with those that produce chemical union, appears abundantly sufficient for explaining some of the most wonderful phenomena of chemistry; such as the prodigious elasticity of steam, of gunpowder, and the still more astonishing explosion of fulminating gold and silver. Some of the phenomena of deflected light are produced by these optical forces acting at distances sufficiently great to admit of measurement; as in the Newtonian observations on the passage of light near the edges of opaque bodies. These deflections enable us to compare the deflecting forces with gravity. The refracting force, however, is vastly greater than even this, as may be seen by the greater deflection which is produced by it; and, being exerted along a space incomparably smaller, it must be greater still. Here, then, are forces fully adequate to the phenomena of fulmination. And we would again desire Mr De la Place to remark that, although these exploding forces are irresistible, their action seems to vanish entirely beyond the limits of mathematical contact. This is plain from the fact, that those explosions do not project the fragments to great distances. This is remarkably the case in all the most eminent of them. Common or nitric gunpowder is perhaps the only great exception. This particular circumstance will surely suggest to this eminent analyst the inverse triplicate ratio of the distance as more likely to explain the phenomena than his favourite law.

We trust that our readers will not be displeased with this short sketch of Dr Herschel's discovery, and the few reflections which it naturally suggested to our minds. We shall not be greatly surprised, although it should produce a sort of counter revolution in chemical science, in consequence of new conceptions which it may give us of the union of bodies with light and heat. The phenomena of the vegetable and animal economy show Thevenot, that they are susceptible of combination with other substances besides the basis of vital air. Whatever changes this may produce in the great revolution which has already taken place in chemical science, they will (in our opinion) be favourable to true philosophy; because Dr. Herchel's discovery co-operates with other arguments of sound mathematical reasoning, to overturn that principle on which De la Place hopes to found his atheistical doctrine of fate and necessity. It contributes therefore to restore to the face of Nature that smiling feature of providential wisdom which Newton had the honour of exhibiting to the view of rational men. The sun is the source of light and genial warmth to a vast system, which is held together, in almost eternal order and beauty, by a law of attraction selected by Infinite Wisdom, as the only one adequate to this magnificent purpose.