386 Mr Smithon on the Discovery of [May, 



5. Arsenical Acid. — Alkali containing it produces a brick-red 

 precipitate with nitrate of silver.* 



6. Chromic Acid. — Chromate of soda and its solution are 

 yellow, and so is the precipitate with lead. That with silver is 

 red. 



Chromate of soda or potash fused on a plate of clay leaves 

 green oxide of chromium. 



Chromate of lead fused on a plate of clay produces a very 

 dark-green mass, which is probably chromate of lead ; with an 

 addition of lead, it forms a fine red, or orange glass. 



Lead added to the green oxide left by chromate of soda on 

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



The green oxide of chromium sometimes acts the part of an 

 acid. 1 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. lxviii. fig. 63) ; melted with 

 lead on the clay plate this would undoubtedly 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 sulphuric 

 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 precipitate. 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 sul- 

 phuric acid, the granules of precipitated tungstic acid become 

 blue, but not the solution ; and the phenomena cannot be con- 

 founded 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 extracting the 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 paper. Possibly a small remainder 

 of muriatic or sulphuric acid among it is required for this effect. 



9. Nitric Acid. — Nitrate of ammonia produces no deflagration 

 when filtering paper, wetted with a solution of it and dried, is 



• Annals of Philosophy, N. & vol. iv. p. 127. 



