{ 341 ) 



by internal vaporlfation ; it Is not neceflary 

 that they fliould fubllme with evident ex- 

 tern?' indications ; this depending on the 

 nature of thefe fubftances ; fome fubhming 

 with one degree of heat, and others requir- 

 ing another and a greater. Hence we may 

 perceive why, in the different experiments 

 in different matraffes, indications were not 

 difcernible of this fubUmation ; though I 

 have no doubt that it would have been ob- 

 tained Ukewife in them, had the fire been 

 fufEciently increafed. 



But to return to the experiments I was 

 relating. After having made thofe now 

 defcribed on volcanic glaifes and enamels, I 

 proceeded to fubjed: to the fame proof a 

 fad:itious glafs ; of that kind which is of a 

 black colour, and has little tranfparency, and 

 which is ufed in many countries to make 

 thick bottles to contain wine. I took one 

 of thefe bottles, which had never had in it 

 any wine or other liquor, and placed twelve 

 ounces of it, as before, in powder, within 

 a matrafs, I had before cbferved that this 

 Z 3 glafs 



