378 PHILOSOPHICAL TRANSACTIONS. [ANNO J 708. 



Ruby. Ruby. 



Quist 3.500 Brazil. Blumenbach . . 3.760 



Brisson 3.531 Brazil. Brisson 3.760 



Klaproth 3.570 Hatchett and G. 4.166 | Sa ^ m *** Star-stone. 



Hatchett and G. 3.571 *Octoedral crystal. \ Coast. 



H. and G 3.625 *Macle of octoedral cryst. Quist 4.200+ 



Blumenbach.. 3.645 Bergman 4.240+ 



Diamond. Diamond. 



Hatchett and G. 3.356 Perfect crystal. Muschenbroeck 3.518 



Wallerius 3 400 La Metherie .. 3.520 



Hatchett and G. 3.471 Aggregate crystal. Brisson 3.521 



Cronstedt 3.500 Werner 3.600 



The mark * distinguishes the specimens in my collection, to which I have re- 

 ferred in the foregoing paper. The mark. -J- distinguishes the stones which, from 

 their specific gravity, I think belong to the genus of corundum. The generic name 

 corundum, I am in the habit of giving to those sorts which have a sparry or a 

 granulated fracture. When corundum has a vitreous cross fracture, I call it sap- 

 phire; and distinguish its varieties by their colours, white, red, blue, yellow, green; 

 and by the accidental reflection of light from their laminae : when in one direction, 

 I call the sapphire chatoyant ; when the reflection is compounded of rays which in- 

 tersect each other, and appear to diverge from a common centre, I call them star- 

 stones, as red, blue, or greyish star- stones, or star-sapphires. 



XX. On the Chemical Properties that have been attributed to Light. By Benjamin 

 Count of Rumford, F. R. S v M. R. L A. p. 449. 



In the 2d part of my 7th essay, on the propagation of heat in fluids, I have 

 mentioned the reasons which had induced me to doubt of the existence of those 

 chemical properties in light that have been attributed to it, and to conclude, that 

 all those visible changes produced in bodies by exposure to the action of the sun's 

 rays, are effected, not by any chemical combination of the matter of light with 

 such bodies, but merely by the heat which is generated, or excited, by the light that 

 is absorbed by them. As the decision of this question is a matter of great import- 

 ance to the advancement of science, and particularly to chemistry, and as the sub- 

 ject is in many respects curious and interesting, it has often employed my thoughts 

 in my leisure hours ; and I have spent much time in endeavouring to contrive 

 experiments, from the unequivocal results of which the truth might be made to ap- 

 pear. Though I have not been so successful in these investigations as I could wish, 

 yet I cannot help flattering myself, that an account of the results of some of my 

 late experiments will be thought sufficiently interesting to merit the attention of 

 the it. s. 



Having found that gold, or silver, might be melted by the heat, invisible to the 

 sight, which exists in the air, at the distance of more than an inch above the point 

 of the flame of a wax-candle, (see my 7th Essay, part 2, page 350,) I was curious 

 to know what effect this heat woud produce on the oxides of those metals. 



