Nitrous Acids on the Sulphocyanides. 231 



of this peculiar red compound may be mistaken for the presence 

 of iron, as, for example, in testing sulphuric, hydrochloric, or 

 nitric acid for iron, as well as in searching for the same metal in 

 the salts of the alkalies, &c, wherein the sulphocyanide of po- 

 tassium (one of our most delicate tests for iron) is used, either 

 alone or in conjunction with hydrochloric, sulphuric, or nitric 

 acid, as the means of detecting that metal ; for the apparent indi- 

 cations of iron so obtained may be only caused by the action of 

 the nitric or nitrous acid used, or present in the other acids em- 

 ployed, or even by the presence of some nitrates or nitrites in 

 the compounds under examination, the nitrogen acids of which, 

 being set free by the sulphuric or hydrochloric acid employed, 

 reacting on the sulphocyanide, may bo the sole cause of the effect 

 which might be so naturally mistaken for iron. I have ascer- 

 tained, however, that there are several points of distinction be- 

 tween the red compound produced by the action of the nitrogen 

 acids on the sulphocyanides, and the sulphocyanide of iron, 

 resembling it so closely in appearance, which may be easily 

 taken advantage of in distinguishing one from the other. 

 Amongst these the following may be mentioned : — 



1st. The former is very unstable, and completely disappears 

 on standing a short time, or immediately on heating it; whereas 

 the latter possesses considerable permanence at the ordinary 

 temperature, and even only very slowly disappears by the con- 

 tinued application of heat. 



2nd. The smallest portion of a solution of the ferrocyanide of 

 of potassium added to a mixture coloured red by the sulphocy- 

 anide of iron instantly changes its colour to a deep blue ; 

 whereas, in the other case, that reagent appears at first to have 

 very little effect on its colour, till it has been added in consider- 

 able quantity, when its dark red coloration gradually passes into 

 a bright yellow, owing apparently to the conversion of the ferro- 

 cyanide into ferridcyanide of potassium ; whilst each drop of the 

 reagent added is attended with the copious evolution of small 

 globules of gas. 



3rd. The alkalies (potash, soda, and ammonia) added in excess, 

 decolorize both red compounds; but this in the case of the metal 

 iron is attended with the precipitation of its oxide, whereas there 

 is no precipitation whatever produced with the other compound. 

 On the addition, however, of an excess of either hydrochloric or 

 sulphuric acid the red coloration is restored in both cases. The 

 above differences will easily enable us to distinguish between 

 these two compounds; but, in addition to them, I may mention 

 another interesting peculiarity I have observed in the case of the 

 compound formed by the nitrogen acids and their salts — that it 

 is very quickly taken up and dissolved by chloroform, ether, 



