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expenfive, and even hammered tin reflectors notvery eafily procurable, a reflector call of Paris platter and gilt on its anterior furface 1 willani'wer very well, and the reflection of heat fromthis cheap inftrument will Hill be much greaterthan from any glafs mirror of equal dimenfions.

The above experiment of Mr. Leflie fhewtherefore a correfpondence between the powerof propelling and abforbing heat: a continuationof the fame refearches throw further light onthe curious queftion how far the propulfion ofheat is affeCted by the nature and condition ofthe propellent furface. A few only of manyexperiments can here be related. With theapparatus of the canifter and mirror above de-fcribed, the effeCt of the plain tin fide in pro-pelling caloric is equivalent to I % degrees of thedifferential thermometer when that of the black-ened fide (affumed as a ftandard) is ioo°. Thetin being rubbed with mercury fo as to beof a dull white, gave 14 0 , but being foakedwith mercury fo as to form almoft a refplendentcoating of this metal, gave 20°. Hence wemay infer that the propellent power of mercuryis about double that of tin..

The effect of the fucceffive degrees of oxy-dation of the fame metal is linking. A cleanrough furface of lead produces 19 0 of the ther-mometer ; the fame, tarnilhed with a grey cruftby expofure to air, gives 45 0 ; a coating ofminium gives 8o°; fealing-wax and rofin eachgives about 95 0 .

The degree of polifh or evennefs of furfacealfo much influences its power of propellingcaloric, which encreafes in proportion as thefurface is roughened. Thus the effeCt of fmoothhammered tin being always 12°, when fcrapedto a bright but irregular furface it amounts to16° ; when ploughed by a fmall-toothed plane-iron, with intervals of of an inch betweenthe teeth, the effect rifes to 19 0 ; when tho-roughly filed over the whole furface it amountsto 26°. Thus if fmooth tinfoil gives 12 0 , thefame rubbed in one direction with fand-papergives 22°. But if the fcratches are much mul-tiplied, an approach to fmooth polifh is made,and the effect is again diminifhed.

The thicknefs of the propellent furface, ormore accurately the thicknefs of a ftratum ofmatter applied to a heated furface and of a dif-ferent nature from it, alfo in fome cafes con-fiderably modifies the propellent power.

A fmall portion of liquefied jelly fpread on afmooth furface of the tin canifter, and dried toa very fine 'iridefcent pellicle, gave 38° (the

effeCt of the unfmeared tin being always 12°) j,a pellicle of four times the thicknefs gave 54°;a thick layer of ifinglafs gave about 8c°. Thisencreafe however has its limits, for after thethicknefs amounts to the thoufandth part of aninch, the propellent power is not encreafed byany fubfequent additions. This rule does not-apply however to any metallic coating appliedto glafs or other metal. The effect in this in-ftance is purely that of the metal at the furface,and is not at all modified by the thicknefs. Thustinfoil applied to glafs appears to have the famepropellent power as a folid plate of tin whofefurface has the fame denfity, fmoothnefs, andpolilh as the foil, and a fingle thicknefs of goldleaf produces the fame effect as any number ofleaves.

The colour of the heated furface appears tomodify in fome degree its receptive and pro-pulfive power with regard to caloric. Thusblack feems to have thefe powers in the higheftdegree, the higheft effeCt in the above experi-ments being produced by a coating of fize andlamp-black, but as the nature of pigments variesextremely, fome being fimply vegetable matter,.and many of them metallic oxyds, it is difficult •to obtain any accurate refults on this fubjedl.Thus, white paper produces nearly the fameeffeCt as lamp-black, and ftained paper nearlythe fame as white;

The modifications of the propulfive and ab-forbent power according to the nature, texture,and ftate of the furface of the propelling, orthe abforbing body, are beautifully illuftrated byextending the fame experiments to the rejleBionof caloric. Here, as already mentioned, therange of the above effeCts will be entirely re-verted, as the furfaces that propel and abforbheat the moft readily are thofe which refleft itwith the greateft difficulty, and vice verfa.

The refleCtive power of different metals mightbe afcertained by caufing reflectors of equaldimenfions to be made of the refpeftive metals,,but as this method would be difficult and ex-penfive, Mr. Leflie ingenioufly. took advantageof the fecondary reflection produced by placinga circular flat plate or dilk of the metal to beexamined, parallel to the face of the tin refleCtor,and nearer than the proper focus. This by thelaws of catoptrics (which apply equally to thepropagation of heat) will produce a fecondaryreflection from the dilk, whofe focus will be-fituated as much before it as the focal point ofthe refleCtor is behind it. Under thefe circmn—fiances the thermometer, whofe bulb was placed.* Pi&et..