218 ESSAY XIX. 



certain quantity of phlogiston, are coagulated to a dry 

 earthy substance, and, on complete saturation, are brought 

 to the state of complete metals. 



The reasons which induced me to suppose that the 

 above-mentioned acid earths are metallic acids, reduced by 

 phlogiston to that form of metallic calxes in which the 

 acid is still perceptible, are the following: 



(1) They both show a striking resemblance to white 

 arsenic in form, in producing effects like acids, and in their 

 little solubility in water. 



(2) Their specific gravity; that of white arsenic is 3750, 

 that of the earth of molybdsena 3460, and that of the 

 acid earth of tungsten about 3000. 



(3) Their precipitation with phlogisticated alkali. None 

 but metallic substances are hitherto known to be pre- 

 cipitated by this alkali. Moreover, arsenic, properly dis- 

 solved in the marine acid, gives with the phlogisticated alkali 

 a precipitate soluble in water, just as the acid earth of 

 tungsten. 



(4) Their property of tinging vitreous fluxes. Metallic 

 calxes, but, as far as I know, no other substances, have the 

 property of communicating a colour, each according to its 

 peculiar kind, to clear glass. The effect of white arsenic 

 in this respect is well known ; that of the earth of 

 tungsten has been described above ; and the earth ex- 

 tracted from molybdama is not less powerful, for it gives 

 to microcosrnic salt a beautiful green colour, and borax 

 well saturated with it appears grey when viewed by the 

 reflected rays, but, by the refracted, of a dark violet 

 colour. 



These reasons have induced me to consider the acid 

 earths obtained from molybdama and tungsten as metallic 

 calxes. Other occupations have hitherto prevented me 



