272 ANNUAL OF SCIENTIFIC DISCOVERT. 



two of a solution of either sulphuret of ammonium, sodium or potassium, 

 when, if nitric acid were present, a fine purple or violet color will be 

 produced, which, however, is not permanent, but soon disappears. In 

 using this test, certain precautions arc necessary, viz, 1st, much dilu- 

 tion of the materials must be avoided, as the acid is required to be 

 tolerably strong to produce thq necessary reaction. The muriatic acid 

 I have used is of specific gravity 1-15 ; if a much weaker acid be em- 

 ployed the results will not be satisfactory. 2nd. If the nitric acid or 

 nitrate be in very minute quantity, the mixture of it with the ferrocy- 

 anide and acid (already referred to) should be allowed to cool to the 

 ordinary temperature before the alkaline carbonate is added ; much 

 excess of this last substance should be avoided, as at a temperature of 

 about 100 it begins to decompose the nitroprusside, on the formation 

 of which the test depends. 



In proof of the delicacy of this test, I may remark that I have de- 

 tected by its means the nitric acid in the one-twohundredth part of a 

 grain of nitre; and with one-hundredth part of a grain the effect is 

 very striking. Its efficacy, too, seems scarcely impaired by the pres- 

 ence of a number of substances, whether of mineral, vegetable, or 

 animal origin, as by it I readily detected the presence of nitric acid in 

 mortars taken from old buildings ; also where that acid or its salts were 

 added in small quantity to soils, sulphuric and muriatic acid, tea, por- 

 ter, milk, &c. 



I may further add, that there is generally a striking difference 

 between the cases where no nitric acid is present, and where it occurs, 

 when the substance is heated with muriatic acid and the ferrocyanide. 

 When nitric acid is not present, the mixture quickly becomes of a 

 blue color ; but where that acid exists, it first becomes of a yellowish 

 green, then of an olive or dark brown ; but these indications alone 

 are not sufficient to prove the presence or absence of nitric acid, until 

 afterwards confirmed by the action of an alkaline sulphuret. In 

 heating the mixture, the temperature stated should be maintained for 

 a few moments till it ceases to acquire a darker shade, thereby indi- 

 cating that all the nitroprusside is formed. 



Sulphuric acid may be substituted for muriatic in using this test ; 

 but I prefer the latter, as being more easily procured pure, and as 

 strong sulphuric acid aided by heat will decompose the nitroprussides. 



This test may also admit of application to nitrous acid and the 

 nitrites ; but as those compounds are comparatively unimportant I will 

 not now enter on this subject. Philosoph. Mag. 



FORMATION OF PROTOXIDE OF NITROGEN BY THE ACTION OF 

 NITRIC ACID ON SAL-AMMONIAC. 



The following method of deriving almost pure protoxide of nitrogen 

 (laughing gas) by the action of nitric acid on sal-ammoniac, is com- 

 municated to Silliman's Journal by Dr. J. Lawrence Smith. 



The experiments made with the nitric acid heated with sal-ammo- 

 niac, to test the character of the decomposition, have resulted in the 



