186 Proceedings of the Royal Irish Academy. 



some freshly prepared fulminate of iron a dilute solution of caustic 

 potash be carefully added, the mistui'e will continue (asl have ohserved) 

 neutral so long as the alkali produces a fiu'ther precipitate of the oxide 

 of iron ; and when it ceases to do so, if the mixture be then filtered, a 

 light yellow solution will be obtained, which holds dissolved, as I 

 conceive, a double neutral fulminate of iron and potassium resulting 

 from the displacement of one half the ii^on in the neutral salt, as is 

 shown in the following equation: 2 Fe aiS^oOo + 2 KHO = Pe K, 

 {Q'^X)) - + Pe + HoO, and this double fulminate develops, as be- 

 fore observed, the piu'ple coloration when treated with a diliited acid, 

 and again becomes colourless, or very nearly so, on adding an excess of 

 alkali, especially after the application of heat, and the colour' can be 

 again restored by acidifying the mixture, and these changes may be 

 produced many times in succession. 



Por the production of the acid fulminate of iron the double neutral 

 salt is much preferable to that of the simple neutral fulminate of that 

 metal, as in the latter case there will be produced, as before shown, 

 a protosalt of iron, which reacts on the acid fulminate, occasioning its 

 more or less rapid decomposition. 



I may further observe, that on heating the double fulminate just 

 described, there will be developed the red or pru'ple coloration, ujiless 

 there is i^resent too great an excess of alkali ; this remark, however, 

 does not apply to the case of ammonia, the excess of which being ex- 

 pelled by heat does not interfere with its production. 



This development of the acid fulminate by heat is not so easily 

 accounted for as where it has been due to the action of acids ; it may, 

 however, depend on the circumstance that the simple neutral, and 

 double fulminate of iron, are both easily resolvable by heat under cer- 

 tain conditions into the acid fulminate. 



I shall now point out how the explanation I have given of the produc- 

 tion of the piu^ple coloration in the case of the fulminate of iron may 

 likewise serve to account for the similar development of colour, which 

 I have myself observed, in the reaction of the f errocyanide of potassium 

 on fulminate of mercury. My experiments would seem to show, that 

 when those compounds react on each other, there 'is at first formed, 

 amongst other products, the double fulminate of iron and potassium, 

 which, Like that salt prepared directly, as ali-eady described, fi'om 

 the fulminate of ii'on, passes into the pui'ple acid fulminate of that 

 metal, on being heated or treated with diluted acids ; the following 

 formulae and equation explaining the production of the double salt, 

 accompanied, as it is in this case, by the cyanide of mercury and potas- 

 sium : 2 Hg Co^^oO. + K4 Fe Cye = Pe K„ (CKOo)- + 2 Hg Cy, + 2 

 KCy. 



Amongst other facts which might be mentioned in support of the 

 foregoing statement, is the following one, that I have observed, that 

 when the ferrocyanide of potassium and the fulminate of mereiuy, 

 along with water, react on each other at the ordinary temperatui'e, the 

 mixture after a short time acquires a yellowish tint, which gradually 



