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however, with Meffrs. Wallerius and de Luc, affert that there is no
degree of heat in the fun's rays. They are active, and therefore
in the collision of their particles may produce fome heat; but from
their extreme tenuity I take that heat to be very fmall. Hence I
am apt to conjecture, that by far the greatest part of the heat pro-
duced by thofe rays is to be attributed to the attraction and repul-
sion between matter effentially active emitted from the fun its
great fource, and that portion of the fame matter originally or ad-
ventitiously incorporated with terreftrial inert matter, which is
thereby put in motion and fermentation. The heat thence refult-
ing will be more or less confiderable, on one hand, in proportion
to the more or lefs direct, divergent, or concentred incidence of
the folar
rays, and the purity of their emanations-and on the other
hand in proportion to the quantity of that fame active fluid con-
tained in the atmospheric particles, or in the bodies acted upon by
thofe rays. Visible terrestrial fire I take to be occafioned by this
active matter, more or lefs firmly united to and imprisoned within
inert matter, producing an explofion by the strong attraction of its
external correfpondent matter. When produced, it carries off with it
all inflammable, oily, humid, and aërial parts containing an abun-
dance of active matter, and leaves behind it, in afhes or refiduum,
fuch earthy particles only as contain very little, and are not fufcep-
tible of expansion and fublimation. I muft obferve that the dia-
mond, which feems not to contain much inflammable matter, but
apparently a great abundance of the element of light, neither flames

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nor appears in a state of ignition or incandefcence in the furnace, but is at length entirely diffipated in the fire by fublimation, without leaving any visible drofs behind it. The ruby has yet proved inattackable by fire. Fire has no other effect on it, but that of somewhat fullying its luftre. Its red rays or colouring principles are too ftrongly fixed to be difperfed by its force.

Whatever may be decided on the question, Whether the folar rays are or are not the immediate caufes of heat, the great Sir Ifaac Newton has fhewn to demonftration that they are not only the cause of colours, but are feparately of diftinct colours. Light excited by fire and flame has the fame quality in that respect apparently, because the element of light in that case shews itself in its original pure ftate. Light feems to be the pure active element, the fource of all motion and life. Earth, air, moisture, and heat, are not fufficient to bring plants to perfection. Light is neceffary to invigorate them, and to bestow on them their proper colours. Experience shows that the fickly flower nurfed in obfcurity will want its varied hues. Light muft either impart the colouring principles, or it must prepare and elaborate its texture to receive or reflect fuch of its rays as will produce to the eye the genuine and vivid colours of its fpecies.

From all that I have faid, you will, Sir, perhaps rather perceive the difficulty of the subject than be able to form any decided opinion on it. Philofophy has not yet attained any fatisfactory expla-nation, and to its future inveftigations I muft leave it.

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(a) Page 436.

WITH respect to this experiment of Meffieurs de Sauffure and Pictet,

I perfectly agree with Dr. Hutton, in a differtation 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 progreffively 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 fome intervening object, from which it again moves in fome 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 inftance 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 obfcure heat which was here reflected and concentrated, but the element of light ftill exifting in its active state on the furface of the cannon ball no longer incandescent, but fo weakened as not to affect our vifion with the fenfation of light?. As he obferves, light affects not every eye alike: to the perfon coming out of broad day-light, 3 M 2


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objects in a darkened room are invisible, which to others who have been long in obfcurity, and to the fame person after ftaying fome time in it, are perfectly difcernible. Light is, in my opinion, the fole effentially active matter, as I fhall hereafter obferve, effluent and refluent from and to the great body of the fun. Abforbed and imprisoned in terreftrial matter, it becomes a constituent part of all other fublances, to which it imparts the colouring principle, but is probably without gravity. Its laws, as well as those of elective attraction and repulsion and of gravity in all terrestrial matter, cannot be refolved but in the will of the Creator. Dr. Hutton judiciously obferves, that the different coloured rays into which light is divided are wifely endowed with different powers with refpect to exciting vifion or heat. The combination of all is the most forcible on vifion, but has the leaft power to excite heat, whilft the properties of the separate rays are inverse as to thefe two purposes. Its power of exciting heat I take to proceed from its attraction of congenial particles abforbed by terreftrial substances, which by their revived activity give motion to such elastic bodies as are compounded of a confiderable 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 progreffive. It is with great pleasure that I find my general ideas on light fo confonant to, and fupported by, thofe of this author, whofe late publication will, I hope, further excite the attention of all philofophers to this important fubject.

(b) Page 447.

The human conception never appears fo finite, the proud lord of this little earth never feems fo truly to fhrink into an atom, as when he raifes his thoughts to the immenfity of creation. From the microscopic infect to thofe innumerable luminaries rolling in that space to which we can affign no limits, all is wonder and astonishment. The effence of even the minutest



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