as on more elevated points where ice and fnow perpetually prevail But on the latter, above all valleys and above the reach of the violent currents of air they occafion, the usual though colder temperature of the air is not adventitiously increased. In the lower Cordeleras the cold currents of wind, rushing impetuously from their narrow, deep, and winding valleys, frequently occafion the inftant death of travellers; yet the general temperature of thofe parts where fnows never lie is very far from being fo cold as that of their higher regions eternally covered with ice and fnow, though situated directly under the line. On lower mountains the penetrating cold of ftrong currents frequently overcomes the influence of the folar rays on human bodies. On mountains isolated by their fuperior height the air, though really colder, is calm, whilft the rays in a purer medium are more active, and confequently have their full effect on our frame. On the whole it feems difficult to pronounce, whether the rays of the fun are effentially hot and the conveyers of heat, or whether they are only the exciters of heat, by giving motion to dormant calorific particles, or accelerating that of fuch as are already in action. Those rays are indeed effentially luminous and the conveyers of light; but the very material differences eftablished by Mr. Wallerius between light and heat make it doubtful whether heat is the neceffary concomitant of light proceeding from those rays. In particular, the propagation of light and heat fo diametrically oppofite moft affuredly affuredly favours the idea of their being diftinct elements. All the phænomena of heat may in all cafes be as eafily, and ́in many cafes. more eafily, (b) explained, by supposing the folar rays not the real conductors but the exciters of heat; and confequently it may be doubted whether light and heat are effentially united.. All terreftrial bodies have more or less active matter, usually called fixed fire, united in their texture. It is perhaps to the pureft particles of this element, effentially the fource of colours, that every fubftance owes its colouring. But the greatest part of this active matter is found in them in an already compofed ftate, forming calorific, inflammable, or phlogistic matters. When pure, I take this active matter to be the element of light, whose great repository in its original state in our fyftem is in the fun, but which in a combined ftate is fcattered throughout all nature. When fixed and imprisoned in inert fubftances, it produces no heat; but when put in motion by the strong attraction or by the contact of its external correfpondent matter, it communicates more or lefs motion to the mert matter to which it is united, and thence produces fermentation and heat. Calorific and inflammable matters contain a much greater abundance of active matter, much more eafily difengaged and put in motion; and, when thefe are ftrongly acted upon, they produce extreme heat, and eventually fire and flame, till they are exhausted. This will, I think, account for all the proceffes of heat, and for the part the folar rays have in exciting it. I will not however, 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 collifion 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 produced by thofe rays is to be attributed to the attraction and repulfion between matter effentially active emitted from the fun its great fource, and that portion of the fame matter originally or adventitiously incorporated with terreftrial inert matter, which is thereby put in motion and fermentation. The heat thence refulting 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 same active fluid contained in the atmospheric particles, or in the bodies acted upon by thofe rays. Vifible terreftrial fire I take to be occafioned by this active matter, more or less 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 abundance of active matter, and leaves behind it, in afhes or refiduum, fuch earthy particles only as contain very little, and are not fufceptible of expansion and fublimation. I muft obferve that the diamond, which feems not to contain much inflammable matter, but apparently a great abundance of the element of light, neither flames nor 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 fomewhat 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 shewn 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 cafe fhews 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 investigations I must leave it. 3 M NOTES |