Communication of Heat, — Light of Ftjhts, £?c, aa t 



fiippufes ** that the cooling caufes miijl mud have a ftronger effecb on the mercury in a 

 fmall bulb ;" but why they (hould have fuch efR:£l, is left for others to difcover. Nor do 

 I think, Sir, that you were happy in the attempt to extricate him from the dilemma. I 

 readily grant that currents of air may afcend, which, though warmer than the encircling 

 mafs of atmofphere, are colder than the body itfelf. But I maintain that thofe currents 

 have already received their heat, and afcend only in confequence of that communication.— 

 The air which touches the hot body receives its full (hare of heat, and, at the fame inftant, 

 ks force of afcenfion .' after it has begun to mount, the efFeft has ceafed ; nor can it exert 

 its cooling energy unlefs it again defcends, to renew the procefs *. To argue otherwife 

 would require the affiftance of a fort of aitiology like that with which fome chemical wri- 

 ters have amufed us of late years concerning pre-difpofing or pre-exiftcnt affinipes. But 

 I will not dwell on a difpute of trivial moment. The author of a periodical work, who is 

 obliged, frequently perhaps without premeditation, to fatisfy the urgent calls of his com- 

 pofitor, is entitled to much indulgence. I moft readily excufe the paffage which was criti-- 

 cifed, and which might inadvertently flip from your pen; and by printing the above, you. 

 will give to the public one proof more of your impartiality and candour. 

 I am, SIR, 



Your moft obedient Servant,. 



JOHN LESLIE^. 



[Erratum-^in laft Number, page. 346, line 9, before vifual read /aw^.] 



VIII.. 



^xpimnenis and Obfervaiiom on the light which is fpontanioufly emitted, with fome-. 

 Degree of Permanency, from various Boit^is. By Nathaniel Hulme, Af.. i}.' 

 F. R. S. and A, S>\ 



INTRODUCTION. . 



X HE difcoveries which have been made with refpe£l to light, as it proceeds immediktely ■ 

 from the fun, are many and. important 5. but the obfervations on tlut fpecles of light 

 which is fpontanepufly emitted from various bodies, ^arc. not only few. in number, but. in 

 general very imperfed. The author is therefore defirous of drawing the future attention ■. 



•The air, which is fii'ft heated, afcends upon- the fame principle as other floating bodies ; namely, be- 

 caufe its fpecific gravity is diminiflied, and the upward prefTur* of .the fluid beneath, is,, therefore, greater 

 than that of the fuper-incumbent column added to that of the heated mals. The lower part cf.ithe whole 

 afcending current (which Mr. L. feems to overlook) is as cold as the reft of the atmofphere ; and it is . 

 ebvioufly thjs portion which maintains the cooling precefs, by (biking the inferior furface of the thenoo- 

 ipeter in its afcent.— N. 



•f Philof, Tranf, 1750, p. i6u. 



