DETECTION OF ARSENIC. 127 



basis, and ask for the analysis of such substances the same 

 application of the general principles of chemical science, 

 which are applied to other analysis. The opinion is enter- 

 tained, that if such were the case, the results of the ana- 

 lyst would be less liable to fallacy, and his opinions less fre- 

 quently subjected to the criticism of the gentlemen of the bar, 

 or the mortifying fact of finding all his inductions contradicted 

 by a professional rival, deemed perhaps by ihe auditory, equal- 

 ly expert and profound. 



Impressed with the importance of these views, I have desig- 

 nated, in a former portion of this essay, the great principles 

 upon which the processes subsequently described were based, 

 and having performed that duty, the propriety of such a course 

 is humbly submitted to the judgment of all those whose studies 

 and pursuits enable them to decide on Its advantages and 

 defects. These general observations are perhaps all that this 

 branch of the subject requires, and I shall now proceed to 

 describe the action of each particular test, the effect which it 

 produces, and the fallacies to which it is liable. They will 

 be described in the order in which they have been used, and 

 first, therefore, the Ammoyiiacal sulphate of copper. The fal- 

 lacies to which this test is liable, are chiefly referrible to the 

 presence of organic matters in the solution to be examined, but 

 if the means be employed to remove them which have been 

 elsewhere indicated, these fallacies cannot occur. A yellowish 

 tint in the fluid under examination, or the reflection of yellow 

 rays will give any precipitate which may follow its use, a 

 greenish colour, and such a precipitate almost invariably fol- 

 lows the addition of the ammoniacal salts of copper to any 

 fluid, even distilled water. The least yellow reflection gives a 

 pea green colour to the blue oxide thus precipitated. 'The 

 operation of this test is prevented by hydrochloric, nitric, sul- 

 phuric, acetic, citric and tartaric acids in excess. These diffi- 

 culties are however obviated by manifest precautions. The 

 muriate, nitrate, and sulphate of ammonia also interfere with it 

 according to Hunefeld.' 'Almost all vegetable and animal infu- 

 sions likewise interfere with its accuracy, as it will strike a 

 green colour where arsenic is not present.' — Christison. 



We do not however recommend this test where arsenic acid 

 is present. It is the arsetiile and not arseniate of copper which 

 has the grass green colour. 



