WRITINGS OF JAMES SMITHSON. 85 



on the clay plate, dissolves it, and forms an orange-coloured 

 glass. 



The green oxide of chromium sometimes acts the part of 

 an acid. I have seen a combination of it with oxide of lead 

 found in Siberia, in regular hexagonal prisms, having the 

 six edges of the terminal face truncated (Haiiy, pi. Ixviii. 

 fig. 63); melted with lead on the clay plate this would un- 

 doubtedly produce the orange glass; and fused with nitrate 

 of potash it would form chromate of potash. 



7. Molybdic Acid. If molybdate of soda or potash, or, I 

 apprehend, any other molybdate, is heated in a drop of sul- 

 phuric acid, the mixture becomes of a most beautiful blue 

 colour, either immediately, or on cooling. 



The solution of molybdate of soda in sulphuric acid 

 affords with martial prussiate of potash, a precipitate of the 

 same colour that copper does. Tincture of galls gives with 

 this acid solution a green precipitate ; but with an alkaline 

 solution of molybdic acid galls produce a fine orange pre- 

 cipitate. If an alkali is put to the green precipitate, it 

 becomes orange ; and if an acid to the orange precipitate, 

 it becomes green. 



8. Tungstic Acid.' If tungstate of soda is heated with 

 sulphuric acid, the granules of precipitated tungstic acid 

 become blue, but not the solution ; and the phenomena can- 

 not be confounded with those presented by molybdate of 

 soda. Martial prussiate of potash has no effect on this acid 

 liquor. 



Tincture of galls put to the solution of tungstate of soda 

 in water does not affect it. On the addition of an acid to 

 this mixture, a brown precipitate forms. 



If tungstate of soda is heated to dryness with a drop of 

 muriatic acid, a yellow mass is left. On extractingthe saline 

 matter by water, yellow acid of tungsten remains. It is 

 readily soluble in carbonate of soda. If taken wet on the 

 blade of a knife, it soon becomes blue. This is made very 

 evident by wiping the blade of the knife with a bit of white 



