ON LAPIS PONDEROSUS 215 



latter precipitate is soluble in water. If a few drops of 

 marine acid be added to a solution of the acid of tungsten 

 in water, and the liquor be spread on polished iron, zinc, 

 or even tin, the acid acquires a beautiful blue colour ; and 

 the same thing happens if these metals be put into the 

 acid. 



SECTION VII. 



As the acid of molybdnena likewise acquires a blue colour 

 from these metals, it might be natural to suppose that the 

 acid of tungsten is the same as that of molybdtena ; but as 

 in other respects it shows very different properties, it must 

 be of a totally different nature. For (1) the acid of 

 molybdsena is volatile, and melts in the fire, 1 which is not 

 the case with the acid of tungsten (Sec. v. (a)). (2) The 

 former of these acids has a stronger attraction for phlogiston, 

 as appears from its combining with sulphur and the change 

 it undergoes on calcination with oil. (3) Lime combined 

 with the acid of molybd?ena is not turned yellow by the acid 

 of nitre, and is readily soluble in it. With tungsten it is 

 quite the contrary. (4) Ponderous earth, combined with 

 the acid of molybdsena, is soluble in water ; but the combina- 

 tion of the ponderous earth with the acid of tungsten is 

 insoluble. And (5) the acid of molybdsena has a weaker 

 attraction for calcareous earth than the acid of tungsten has ; 

 for if the combination of lime and acid of molybdrena be 

 digested in a solution of the above-mentioned ammoniacal 

 salt (Sec. v. (a)), a regenerated tungsten will be obtained. 



The iron procured from some sorts of tungsten is to be 

 considered as accidentally present. 



1 See the Dissertation on Molybdsena. 



