1821.] the Composition of Prussiates. lL 



several double cyanurets of iron, copper, silver, and gold, with 

 other metals, and it appeared to me very probable, that the cya- 

 nuret of the electro-negative metal might act as an acid with 

 respect to the electro-positive metal, which represents the base. 

 The sulphuretted hydrocyanic acid might very well be a double 

 cyanuret of sulphur and hydrogen ; and in this way the theory of 

 these compounds became simple and analogous to that of the 

 oxides. I afterwards examined the sulphuretted hydrocyanic 

 acid, and I found that its nature was altogether different ; and I 

 shall soon have occasion to mention it to the Academy. As to 

 the idea of regarding the compounds with sulphuric acid as sul- 

 phates of cyanurets, the experiment with prussian blue is opposed 

 to it ; and that which I am going to relate immediately, decides, 

 as it appears, the question in the negative. I took some hydro- 

 cyanate of iron which had been dried in vacuo, and I treated it 

 with sulphuric acid. It dissolved, and produced a colourless, 

 and limpid liquid, which gave no trace of hydrocyanic smell. 

 When exposed to the air, the sulphuric acid deposited a white 

 substance, which I separated from the acid liquor, by means of 

 an absorbing brick. The compound of sulphuric acid with the 

 superhydrocyanate of iron remains in the form of an uncrystalline 

 powder. It is totally soluble in water, which afterwards contains 

 sulphuric acid and superhydrocyanate of iron, which decom- 

 poses by exposure to the air, as if no sulphuric acid were pre- 

 sent : this then is a proof that water has separated them. 

 As in this compound no doubt can exist of the sulphuric acid 

 being combined with hydrocyanic acid, it appears to me to 

 be decided, that the compounds of which we have been speaking 

 are in reality acidulous double salts, in which two bases are 

 combined with an excess of two acids. 



VII. Observations on the Preparation of the Alkaline Cyanurets 

 by Means of Prussian Blue. 



If the prussian blue of commerce be treated with caustic pot- 

 ash in excess, we obtain, after the crystallization of cyanuret of 

 iron and potassium, a syrupy mother water, which refuses to 

 crystallize, but which, when slowly evaporated, effloresces in 

 greenish vegetations. If the excess of potash in the mother- 

 water be neutralized with acetic acid, and if alcohol be after- 

 wards added, it separates a mass of a deep green colour. It is 

 a peculiar modification of cyanuret of iron and potassium, which 

 dissolves in water, and gives a meadow-green colour, but by 

 long exposure to moist air, it becomes brownish. It does not 

 crystallize by evaporation, but it deposits small green scales, 

 especially at the edges of the liquid. The colour of these scales 

 becomes paler, and brownish when they are dried. I analyzed 

 them, and they differed so little from the yellow cyanuret that I 

 could draw no conclusions from the analysis. These differences 

 are derived from a peculiar modification of cyanogen, which 

 exists in prussian blue. While decomposing, this cyanuret 



