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nor appears in a state of ignition or incandescence in the furnace, but is at length entirely diffipated in the fire by sublimation, without leaving any visible dross behind it. The ruby has yet proved inattackable by fire. Fire has no other effect on it, but that of somewhat sullying its lustre. Its red rays or colouring principles are too strongly fixed to be dispersed by its force.
may be decided on the question, Whether the solar rays are or are not the immediate causes of heat, the great Sir Isaac Newton has shewn to demonstration that they are not only the cause of colours, but are separately of distinct colours. Light excited by fire and Aame has the same quality in that respect apparently, because the element of light in that case shews itself in its original pure state. Light seems to be the pure active element, the source of all motion and life. Earth, air, moisture, and heat, are not sufficient to bring plants to perfection. Light is necessary to invigorate them, and to bestow on them their proper colours. Experience shows that the sickly flower nursed in obscurity will want its varied hues. Light must either impart the colouring principles, or it must prepare and elaborate its texture to receive or reflect such of its rays as will produce to the eye the genuine and vivid colours of its species.
From all that I have said, you will, Sir, perhaps rather perceive the difficulty of the subject than be able to form any decided opinion on it. Philosophy has not yet attained any satisfactory expla-nation, and to its future investigations I must leave it.
L E T T E R V.
(a) Page 436. WITH respect to this experiment of Meffieurs de Saussure and Pietet, I perfectly agree with Dr. Hutton, in a dissertation just published, that it could not be heat which was thus irradiated, reflected, and concentrated. We know from constant experience, that heat is circularly and progressively expanfive in all directions, tending to put itself in equilibrium in whatever space it gradually fills, and to place all bodies which it meets in its progress in equilibrium with respect to that quality. Light, on the contrary, we know from every other experiment to move in a direct line till repelled and diverged by some intervening object, from which it again 'moves in some other direct line, and is thus capable of being concentrated in a focus. Are we from this - folitary experiment to conclude that heat in this instance has deviated from its general mode of propagation, and taken up the direct contrary one of light ? - Is it not more reasonable to suppose, with Dr. Hutton, that it was not obscure heat which was here reflected and concentrated, but the element of light still existing in its active state on the surface of the cannon ball no longer incandescent, but so weakened as not to affect our vision with the sensation of light?. As he observes, light affects not cvery eye alike : to the person coming out of broad day-light, objects in a darkened room are invisible, which to others who have been long in obscurity, and to the same person after staying fome time in it, are perfectly discernible. Light is, in my opinion, the sole essentially active matter, as I shall hereafter obferve, effluent and refluent froin and to the great body of the sun. Absorbed and imprisoned in terrestrial matter, it becomes a constituent part of all other subhances, to which it imparts the colouring principle, but is probably without gravity. Its laws, as well as those of ele&tivé attraction and repullion and of gravity in all terrestrial matter, cannot be resolved but in the will of the Creator. Dr. Hutton judiciously observes, that the different coloured rays into which light is divided are wisely endowed with different powers with respect to exciting vision or heat. The combination of all is the most forcible on vision, but has the least power to excite heat, whilst the properties of the separate rays are inverse as to these two purposes. Its power of exciting heat I take to proceed from its attraction of congenial particles absorbed by terrestrial substances, which by their revived activity give motion to such elastic bodies as are compounded of a considerable proportion of its own principle. By the primary laws of nature which we can discover, but which we cannot further investigate, the pure element of light invariably moves in a direct line with inconceivable rapidity; but elastic bodies, though containing the greatest quantity of its principles, obey a different law, whence the propagation of heat is circular and progressive. It is with great pleasure that I find my general ideas on light so consonant to, and supported by, those of this author, whofe late publication will, I hope, further excite the attention of all philofophers to this important subject.
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(b) Page 447 The human conception never appears fo finite, the proud lord of this little earth never seems so truly to thrink into an atom, as when he raises his thoughts to the immensity of creation. From the microscopic insect to those innumerable luminaries rolling in that space to which we can aflign no limits, all is wonder and astonishment. The essence of even the minutest
particle is beyond the utmost sagacity and penetration of man. Struck with so much of the admirable structure both of the whole and of each part as he is permitted to see into, he must confess that more remains behind above his feeble comprehension, and his final exclamation must be:
These are thy glorious works, Parent of Good !
Those innumerable stars which our weak fight can discern, those which the curious astronomer has been able to discover by the help of telescopes, and those, perhaps in still greater number, which even Mr. Herschel's inftruments have not been able to draw within our ken, are all so many suns, many of as great, and possibly many of much greater, magnitude than ours. Reasonable and just analogy pronounces that each of these is probably accompanied by as many attendant planets as our sun. In our fystem the size of this our globe is not even a medium with that of other planets, Whilst the diameter of the Earth is about 6700 miles, that of Mercury is indeed only 2748, but that of Jupiter is 120,650 miles. Can it be supposed that bodies of such magnitude, and stars not inferior to our fun, were created merely to regulate our seafons, to guide our steps or our navigation, or to ornament the canopy of man's residence as with so many lamps? Much less bodies placed at finaller distances would have equally answered these ends. No; such immense bodies, and so many suns, many of which are imperceptible to the naked eye, must have had more immediate and more important purposes. Reason must assure us that the planets of our system, and those attendant on other suns, must have been destined for the habitation of other creatures, of whose natures we are indeed and must remain for ever ignorant, but whose existence seems not problematical. The satellites of some of our planets, so wisely increased and distributed to fupply light at their great distances from the sun, proclaiın it beyond a doubt. If heat, however, is entirely dependant on proximity to the sun and on the intensity of its rays, we can have no conception that any living creature can exist in Mercury, from the violent heat occasioned by the