-404. Cn the Chemical Properties aiirihuied to Light. 



•means of charcoal, (hows that, at a certain (high) temperature, oxygen Is difpofed to quit 

 thofe metals, in order to form a chemical union with the charcoal, or at leaft with fome 

 one of its conftituent principles, if it be a compound fubftance ; and hence I concluded, 

 ■that gold might be revived, i/i the moifl -way, by means of charcoal, from a folution of its 

 oxide in water, were it poffible, under fuch circumftances, to communicate to the charcoal, 

 and to the oxide, at the fame time, a degree of heat fufficient for that purpofe. 



To fee if this might not be done by means of light, I made, or rather repeated, the 

 following very interefting experiment : 



Experiment No, 9. Into a thin tube of very fine colourlefs glafs, 10 inches long, and -n» 

 of an inch in diameter, clofed hermetically at its lower end, I put as many pieces of 

 charcoal, about the fize of large peas, as filled the tube to the height of two inches ; and, 

 having poured on them as much of the aqueous folution of nitro-muriate of gold as nearly 

 covered them, expofed the tube, with its contents, to the adiion of the dire£t rays of a 

 very bright fun. 



In lefs than half an hour, fmall fpecks of revived gold, in all Its metallic fplcndour, 

 began to make their appearance here and there on the furf'ace of the charcoal ; and, in fix 

 hours, the folution, which at firft was of a bright yellow colour, became perfeflly 

 colourlefs, and as clear and transparent as the purest water. 



The furface of the charcoal was, in fevcral places, nearly covered with fmall particles 

 of revived gold ; and the infide of the glafs tube, in that part where it was in contadl: with 

 the upper furface of the contained liquid, was moft: beautifully gilded. 



This gilding of the tube was very fplendid, when viewed by refledled light ; but, when 

 the tube was placed between the light and the eye, it appeared like a thin cloud, of a 

 grecnifli blue colour, without the fmalleft appearance of any metallic fplendour. 



From the colour, and apparent denfity of this cloud, I was induced to conclude, that 

 the gilding on the glafs was lefs than one millionth part of an inch in thicknefs. 



This interefting experiment was repeated fix times, and always with nearly the fame 

 refult. The gold was completely revived in each of them, and the folution left perfedtly 

 colourlefs : in moft of the experiments, however, the fides of the _glafs were not gilded, all 

 the revived gold remaining attached to the furface of the charcoal. 



In two of thefe experiments, I made ufe of pieces of charcoal which had been previoully 

 boiled feveral hours in a large quantity of diftilled water, and which were introduced luet, 

 ■znAhot, into the tube, and immediately covered by the folution, to prevent them from im- 

 bibing any air ; and, in different experiments, the folution was ufed of different degrees of 

 ftrength. 



I plainly perceived that the experiment fucceeded beft, that is to fay, that the gold was 

 fooneji revived, in thofe cafes in which the folution was moji diluted : one of the experiments, 

 however, and which fucceeded perfectly, was made with the folution fo much condenfed, 

 that It was nearly at the point at which it became difpofed to cryftallize *. 



* This agrees perfcflly with the refults of fimilar experiments made by the ingenious and lively 

 Mrs. FuLHAME. (See her Effay on Combuftion, page 124.) 



It was on reading her book that I was induced to engage in thefe inveftigations j and it was by her experi- 

 ments that raoft of the foregoing experiments were fuggefted. R. 



7 On 



