166 ESSAY XII. 



SECTION II. 



I both digested and boiled this mineral in all the known 

 acids ; but among them all I found but two which have any 

 effect upon it, viz. the acid of arsenic and the acid of nitre. 

 The molybdrena is not attacked by the acid of arsenic till the 

 water is evaporated. If then the heat be increased a little, 

 arsenic rises into the neck of the retort, and towards the end 

 yellow arsenic or orpiment is sublimed. Volatile sulphureous 

 acid goes over into the receiver. 



I poured two parts of concentrated nitrous acid upon 

 one part of powdered molybdsena. The mixture was hardly 

 lukewarm in the retort, when it passed all together into the 

 recipient with great heat, in the form of dark red vapours. 

 I doubt not but this mixture would have taken fire if its 

 quantity had been larger ; and therefore thought it more 

 advisable to repeat the experiment with diluted nitrous 

 acid. 



SECTION III. 



I poured 6 oz. of diluted acid of nitre on li oz. of 

 powdered molybdtena, and put it into a glass retort, provided 

 with a luted recipient, and placed it in a sand-bath. During 

 the digestion the acid had no effect upon the powder ; but 

 as soon as the mixture began to boil, red elastic vapours rose 

 with a great intumescence ; the retort therefore ought to 

 be taken large enough. The distillation was continued to 

 dryness. Upon the residuum in the retort, which was now 

 of a grey colour, I poured the same quantity of diluted acid 

 of nitre, when the mass began to effervesce considerably as 

 before. After the mixture was again distilled to dryness, 

 the residuum appeared to be of a whiter colour than before. 



