354 Tests for Arsenic, 



in pencil — " From the number of impressions taken from e 

 similar plate to this, we have no doubt of obtaining two 

 hundred thousand jproo/ impressions from this plate." 



When it is recollected that the common number of proof 

 impressions is two thousand, and that one plate scarcely 

 affords more than four or five thousand prints, Mr. Perkins's 

 invention may well be regarded as one of great value, and 

 as enhancing, in a high degree, the importance of the pro- 

 ductions of the pencil and the graver, so that statuary has 

 scarcely any superiority in this respect, while it has no 

 means of multiplying copies, except the slow and tedious 

 ones by which the originals are produced, or the substitu- 

 tion of plaster casts. 



Art. XVIII. — Tests for Arsenic. — Editor. 



It is a question very interesting to medical jurisprudence, 

 whether there is any test for arsenic, which can be implicitly 

 rehed on, to such an extent as to justify, on that ground 

 alone, the condemnation of an accused person. Some ex- 

 perience, in such cases, has produced in us an increasing 

 impression, that nothing short of the actual production of 

 the metallic arsenic can be safely relied on for the above 

 purpose, although various tests may serve, more or less per- 

 fectly, to guide the inquiries, and to influence the opinion 

 of the practical chemist. 



A pupil. Dr. T. D. Porter, now a member of the faculty 

 of the University of South Carolina, in his inaugural disser- 

 tation states, that he finds, on repeating some of the popular 

 experiments, with onion juice, which were some time since 

 pubHshed in the newspapers ; — that the onion juice, with 

 the solution of sulphate of copper, (blue vitriol,) but ivithout 

 the carbonate of potash, produces, in a weak arsenical solu- 

 tion, " a shade like Scheele's green ;" but, if carbonate of 

 potash be added, the effect is completely different. Con- 

 sidering Scheele's green as a test that has been much relied 

 on for the discovery of arsenic. Dr. Porter formed it in the 

 usual way, with sulphate of copper and subcarbonate of 

 potash; in one experiment, a decided precipitate was pro- 

 duced from a stronger, and, in another, a scarcely percepti- 



