Davy — On Properties possessed by Salts of Fulminic Acid. 185 



ration just stated. My father also obseryed that alkalies produced in 

 freshly prepared fulminate of iron a dull green precipitate, quickly 

 changing to a brown colour, which is obviously due to the separation 

 of iron as an oxide from the f ahninate of iron. And I have myself 

 observed that the light yellow liquid which remains after the action of 

 the alkalies and the separation of the oxide, at once develops a fine 

 port- wine colour, when it is treated with diluted acids, and that the 

 coloration so produced is much more stable than that developed by 

 directly treating the fulminate of iron similarly ; and that this red or 

 purple-coloured compound resembles in all its characters that produced 

 in the new reaction of the ferrocyanide on the fulminate of mercury 

 already referred to. 



As to the singular development of colour when the fulminate of iron 

 is treated with dilute acids, I am not aware that any explanation has as 

 yetbeen given ; and the one that I wouldnow suggest accounts for its pro- 

 duction, not only in the case of the fulminate of iron, but also in the- new 

 reactions which I have myseK recently observed ; and explains some of 

 the properties of this curious purple-coloured compound. To make 

 the explanation I would ofEer intelligible, I should first observe that 

 fulminic acid is generally regarded as a bibasic acid, which is capable 

 of forming two classes of salts, viz., the neutral and the acid salts. In 

 the first, the two atoms of hydi'ogen in the hydrated acid (II2C2N2O2), 

 are replaced either by two atoms of a monad metal, as in the case 

 of the fulminate of silver (AgjCoNjOs), or by one atom of a dyad 

 metal, as in the fulminate of mercury (HgC^jSTaOo). In the second class 

 we have either one atom of hydrogen still retained, whilst the other is 

 replaced by a monad metal, as in the case of the acid fulminate of 

 silver (AgHCaN^Oo), or two atoms of hydrogen are retained (the mole- 

 cule of fulminic acid being doubled) where a dyad metal occurs, as 

 in the acid fulminate of mercury (Hglla {G^^.Jdz)'). l^ow as iron 

 in most of its combinations plays the part of a dyad, we should express 

 its neutral fulminate thus, Pe C2N2O2 ; and when this salt is treated 

 with a diluted acid there is foi-med, as I conceive, an acid fulminate of 

 iron (a hitherto undescribed salt), by the following reaction, where for 

 example, sulphuric acid has been employed, 2 Ee 02^^202 + H2S04 = re 

 II2 (03^202)^ + Fc SO4, and that it is this acid fulminate which pos- 

 sesses the red or purple colour, whilst it is at the same time much 

 more stable or less prone to decompose than the neutral salt. If this 

 acid fulminate is treated with an alkali, its purple colour disappears, 

 owing, as I conceive, to the formation of a neutral double fulminate of 

 iron and the metal of the alkali, which is a coloiu'less salt in dilute 

 solution ; thus in the case of potash being added to the acid fulminate 

 of iron, there would be a double neutral fulminate of iron and potas- 

 sium formed, according to the following reaction : Fe II2 (O^iOiy' + 2 

 KHO = Pe K2 (aWjO,) 2 + 2 H2O, and this colourless solution being 

 treated with a diluted acid a,gain develops the purple colour by the 

 reformation of the acid fulminate, as the following equation indicates : 

 Pc K, (C2IT2O2)' + H2SO, = Fe H2 (02^^02)^ + KjSOi. Or again, if to 



E. I. A. PEOC, SEE. II., VOL. II., SCIENCE. 2 C 



