VOL. LXXXVIII.] PHILOSOPHICAL TRANSACTIONS. 383 



experiment succeeded best, that is to say, that the gold was soonest revived, in 

 those cases in which the solution was most diluted : one of the experiments how- 

 ever, and which succeeded perfectly, was made with the solution so much con- 

 densed, that it was nearly at the point at which it became disposed to crystallize *. 

 On examining, with a good microscope, the particles of revived gold which remained 

 attached to the surface of the charcoal, after it had been dried, I found them to 

 consist of an infinite number of small scales, separated from each other ; not very 

 highly polished, but possessing the true metallic splendour, and a very deep and rich 

 gold colour. The gold which attached itself to the inside of the glass tube, was in 

 the form of a ring, about -^. of an inch wide, badly defined however below, and 

 adhered to the glass with so much obstinacy, as not to be removed by rincing out 

 the tube a great number of times with water ; it had, as has already been observed, 

 a very high polish, when seen by reflected light. Those who enter into the spirit 

 of these investigations, will easily imagine how impatient I must have been, after 

 seeing the results of these experiments, to find out whether gold could be revived 

 from this aqueous solution of its oxide by means of charcoal, without the assist- 

 ance of light, and merely by such a degree of equal heat as could be given to it in 

 the dark. To determine that important question, the following experiment was 

 made. 



Exper. 10. A cylindrical glass tube, ~ of an inch in diameter, and 10 

 inches long, closed hermetically at its lower end, and containing a quantity of a 

 diluted aqueous solution of the oxide of gold, mixed with charcoal in broken 

 pieces, about the size of large peas, was put into a fit cylindrical tin case, which 

 was nicely closed with a fit cover ; and the glass tube, with its contents, so shut up 

 in the dark, was exposed 2 hours, in the temperature of 210° of Fahrenheit's scale. 

 On taking the glass tube out of its tin case, I found the solution perfectly colour- 

 less, and the revived gold adhering to the surface of the charcoal. On repeating 

 the experiment, and using the solution nearly saturated with the oxide, the result 

 Was precisely the same ; the solution being found perfectly colourless, and the re- 

 vived gold adhering to the surface of the charcoal. 



I own fairly, that the results of these experiments were quite contrary to my 

 expectations, and that I am not able to reconcile them with my hypothesis, re- 

 specting the causes of the reduction of the oxide, in the foregoing experiments ; 

 but whatever may be the fate of this, or of any other hypothesis of mine, I hope 

 and trust that I never shall be so weak as to feel pain at the discovery of truth, 

 however contrary it may be to my expectations ; and still less, to feel a secret wish 

 to suppress experiments, merely because their results militate against any specula- 

 tive opinions. It is proper I should observe, that the charcoal used in this last- 



* This agrees perfectly with the results of similar experiments made by the ingenious and lively Mrs. 

 Fulhame. See her Essay on Combustion, page 124. It was on reading her book, that I was induced 

 to engage in these investigations ; and it was by her experiments, that most of the foregoing experi- 

 ments were suggested. — Orig. 



