Arsenic does not blacken a knife by which it is cut, as stated on the trial of _Eliza Fenning_; nor does it, when mixed with dough, prevent its rising.[259] We have now concluded our history of the different tests which have been proposed for the detection of arsenic. Much has been said and written upon the relative degree of confidence to which they are respectively entitled, and it has been asserted on several occasions, that nothing short of the reproduction of the metal ought to be received by the tribunals of justice, as an unequivocal proof of the presence of arsenious acid. (See _Dr. Neale’s Evidence on the trial of Donnall_. _Appendix, p._ 297.) In taking an impartial review of all the evidence which the investigation of this subject can furnish, it must appear to the most fastidious, that the _Silver_ and _Copper_ tests, above described, are capable, under proper management and precaution, of furnishing striking and infallible indications; and that in most cases they will be equally conclusive, and in some even more satisfactory in their results, than the metallic reproduction upon which so much stress has been laid; and for this obvious reason, that unless the quantity of metal be considerable, its metallic splendour and appearance is often very ambiguous and questionable. The author is personally acquainted with a case, where the medical person, by no means deficient in chemical address, actually ascribed the presence of arsenic to that which was no other than a film of finely divided charcoal: in this state of doubt the last resource was to ascertain whether it yielded, or not, upon being volatilized, an alliaceous odour. Surely an unprejudiced judge would prefer the evidence of _sight_, as furnished by the tests, to that of _smell_, as afforded in the experiment to which we allude; especially after the various fallacies, which we have shewn in the course of the present enquiry, to have occurred with regard to this latter sense. But the question at issue may be easily disposed of to the satisfaction of all parties; for let it be remembered, that the application of chemical reagents on solutions suspected to contain arsenic, so far from throwing any obstacle in the way of the _metallic reproduction_ of that substance, are the very steps which should be adopted as preparatory to the “_experimentum crucis_.” It is only necessary to collect the precipitates, and to decompose them in the manner already described; and this confirmation of our results should never be neglected, for it is the bounden duty of the forensic chemist, who is called upon to decide so important a question as the presence of a corrosive poison, to prosecute by the fullest enquiry every point which admits of the least doubt; he should also remember that in a criminal case, where the life of a human being depends upon his testimony, he has not only to satisfy his own conscience, but that he is bound, as far as he is able, to