JI4€ Otfifvatiani en the Chemical JiSiions of Light and Heat, 



This part was become quite black, but contained no globules. This was the more fur- 

 prizing, as I had not obferved any thing fimilar in my former experiments. By accurate 

 examination it was found, that the rays of light had been admitted to this part of the 

 > oxide through the flip of window glafs, which was of a greinijh cajt. Having no prlfm by 

 me, I had not an opportunity of afcertaining the tStCt of different coloured rays. 



As fome of the Count's experiments fcem to indicate, that moifture has fome concern 

 In producing the changes, and having myfelf frequently obferved a fmall quantity of vapour 

 exhale, immediately on expofure to the rays; i endeavoured to afcertain the truth by 

 experiment. For this purpofe 1 introduced into a very dry glafs tube, a fmall quantity of 

 the red oxide made perfeSly dry, and enclofed it in fuch a manner as totally to prevent 

 the accefs of air and moifture. By expofure to the rays the fame changes t6ok place, and 

 the only difference I could perceive was, that in this the oxide acquired its deepeft colour 

 In lefs time than in the former experiment ; moifture then, we may conclude, has no effedt 

 in the redudtion of mercury. 



From thefe experiments it appears, that light a£ls intirely on the furface. This alone 

 feems to be a material objection to the conclufion of the Count, for we cannot well fup- 

 pofe, that fo intenfe a heat as is necefl'ary for the reduction of the oxide, Ihould produce 

 its effefts merely on the furface, and no where elfe. However, there are other objections 

 which appear to be infuperable. If the effeft be produced by the l^eat alone which the 

 aftion of light occafions, it muft neceffarily follow, that the metallic oxides in general* 

 when placed in contact with inflammable bodies, will be reduced by mere expofure to the 

 fun's rays. To know whether this is a£lually the cafe, I made choice of the oxides of lead 

 for the fubjeft of experiment, on account of their well known property of being eafily 

 Tevived by caloric, when mixed with inflammable bodies. Small quantities of minium^ 

 and of acetite of lead, were feparately expofed on glafs to the diredt rays of the fun foy 

 feveral hours. No change was produced, except that the water of cryftallization had ex- 

 haled from the latter. Next day a fmall quantity of charcoal was thinly fprinkled on each, 

 and gently prefled down. After expofure to the rays the whole day, not the fmalleft 

 change was effedted ; they were then put into two fmall cylindrical glafs veflels, and alcohol 

 poured over them to the depth of half an inch, and again expofed on the third day till the 

 fpirit was intirely evaporated, but without any alteration whatever.. 



May we tiot therefore conclude that light adls independent of heat> 



I am, SIR, 



Tour's refpe£lfully, 



ROBERT HARRUP. 

 Cohham, Surry., Sept. i, i8or. 



P. S. I cannot negle£t this opportunity of giving the refult of fome experiments made 

 three years ago, on the production of gas by freezing water, and of which I was reminded 

 at prcfentby reading Dr. Piiellley's communication in the 4th volume of your Journal, 



If 



