294 CHEMICAL EFFECTS OT LIGHT. 



My reafons for withholding Ihefe experiments at that time 

 were, that they appeared foraevvhat irrelevant to the primary 

 fubjed under difcufiion, and that I was alfo in hopes of in- 

 creafing their value by additional trials on otiier fubfiances. 

 Whether the Upon confidering the power which thefe rays poflfefs of ex- 



folar r.iys tend to pell;ng OKygen from the muriate of filver, I thought it not im- 

 ~nof proved? ' P^^^l'^^e, that there might be more truth than I had been ac- 

 curtomed to fuppofe, in the popular obfervation that the fun 

 has a tendency to extinguith fire, as the fame rays might re- 

 tard combuftion by oppofing the abforptlon of oxygen. Ac- 

 cordingly I made various experiments on different fubftances in 

 a ftate of flovv combufiion, but without any apparent con- 

 firmation of lliat hvpothefis. 

 Lighter folar J alfo tried theaftion of light on feveral vegetable blue co- 



fedl vFeetabie " Io'"'s» vvliich are knov,'n to be affeS.ed by union with oxygen, 

 bluesj— and upon the fame colours previoufly reddened; but on thefe 



alfo I did not fucceed in producing any eiFc;£l at either bound* 

 aryof theprifmaticfpeflrum. 

 Trul of guaja- After failure of thefe endeavours, I had recourfe to guaia-r 

 ^"™' cum, which I had long known to acquire a green colour by 



exjiofure to light; but that the prefence of air is alfo requifife 

 for this purpofe I had afcertained in the following manner. 

 No change by Two plates of glafs were heated with a fmall piece of guaia-? 



011^1"^^ ^' cum interpofed, and thereby cemented together in their cen- 

 ters for a circular fpace about If inch diameter. In this ftate 

 the)7 were expofed for feveral weeks during fummer to the fun^ 

 wiliiout the fmallefl apparent alteration in the colour of the 

 guaiacum. 

 .^but when the The plates were then forcibly feparated; and as they were 

 air had accefs j {^qj]^ fimilarly coated with a portion of the refin adhering to 

 their furfacgs, one of them was preferved for comparifon in a 

 dark place, where it had free accefs of air alone, while the 

 Oilier wasagairi expofed uncovered to the mid-day fijn. 

 —.it was afFeft. The latter was in five minutes perceptibly rendered green, 

 ed in five mi- and in a few hours had acquired the full colour, which it feemed 

 noon-day fun. capabje of receiving; but the former, in the courfe of many 

 months that it was kept confined frojin the light, feemed not to 

 have been difcernibly altered. 

 Theprifmatic Since by later experiments it appeared probable, that the 



fp'drum was , 1 /• 1 r . i f-i- • I • /f . . 



too wealc to fbnw whole of the fun's rays were not adive in this procefs, with a 

 whether all the yjew to determine on what part of them the effe6\ might de- 

 rays were effed- -^ J ■ 



ive or the con- pcnd, 



trary, . • ' • 



