Nitric Acid and the Nitrates, 331 



most of them are objectionable, being inconclusive if taken alone, 

 inapplicable in many cases, or requiring the aid of expensive 

 alkaloids not easily procured. The test I propose appears to 

 me to be free from those objections, and accomplishes its object 

 with considerable facility; it depends on the formation of that 

 interesting class of salts lately discovered by Dr. Playfair, the 

 Nitroprmsides, and the beautiful and characteristic effect produced 

 on them by an alkaline sulphuret. I have found that the nitro- 

 prussides may be formed (under certain circumstances) when 

 only very minute quantities of nitric acid or its salts are present. 

 The method I adopt is as follows : — To the solid or liquid sup- 

 posed to contain nitric acid, in its free or combined state, add a 

 few drops of a strong aqueous solution of ferrocyanide of potas- 

 sium (yellow prussiate of potash), then some pure muriatic acid, 

 mixing the ingredients well together both before and after the 

 addition of the acid, and gradually raise the temperature of the 

 mixture to about 160° Fahr., or in some cases it may be neces- 

 sary to raise it a little higher ; let it cool, and then neutralize 

 with carbonate of soda or potash, a slight excess of either being 

 of no consequence. Filter if there is much precipitate, and 

 finally add to the fluid a drop or two of a solution of either sul- 

 phuret of ammonium, sodium or potassium, when if nitric acid 

 were present, a fine purple or violet colour will be produced, 

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

 this test certain precautions are necessary, viz. 1st, much dilu- 

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

 be tolerably strong to produce the necessary reaction. The 

 muriatic acid I have used is of spec. grav. 1*15 ; if a much 

 weaker acid be employed, the results will not be satisfactory. 

 2nd. If the nitric acid or nitrate be in very minute quantity, the 

 mixture of it with the ferrocyanide 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 I have de- 

 tected by its means the nitric acid in the one two-hundredth 

 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 presence 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, porter, milk, &c. 



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



Z2 



