82 An Inquiry concerning the 



As the colouring substance is the same, and as the 

 colours produced are the same, why should we not 

 conclude that the effects are produced in both these 

 cases by the same means, that is to say, by the 

 agency of heat ? or, in other words, and to be more ex- 

 plicit, by exposing the oxide in a certain temperature, 

 at which it becomes disposed to vitrify or to undergo a 

 change in regard to the quantity of oxygen with which 

 it is combined ? 



But the results of the following experiments afford 

 still more satisfactory information respecting the in- 

 tensity of the heat generated in all cases where light is 

 absorbed, and the striking effects which under certain 

 circumstances it is capable of producing. 



The facility with which most of the metallic oxides 

 are reduced, in the dry way, by means of charcoal, 

 shows that, at a certain (high) temperature, oxygen is 

 disposed to quit those metals, in order to form a chem- 

 ical union with the charcoal, or at least with some 

 one of its constituent principles, if it be a compound 

 substance ; and hence I concluded that gold might be 

 revived, in the moist way, by means of charcoal, from 

 a solution of its oxide in water, were it possible under 

 such circumstances to communicate to the charcoal 

 and to the oxide at the same time a degree of heat 

 sufficient for that purpose. 



To see if this might not be done by means of light, 

 I made, or rather repeated, the following very interest- 

 ing experiment : 



Experiment No. 9. Into a thin tube of very fine 

 colourless glass, 10 inches long and & of an inch in 

 diameter, closed hermetically at its lower end, I put as 

 many pieces of charcoal, about the size of large peas, 



