This method of investigation gives extremely precise results, because it presents the poison successively in three distinct forms, as sulphuret, metal, and crystallized oxide, all of which possess very prominent and characteristic external properties. It is also a method which is capable of detecting very minute quantities of oxide of arsenic. And it has the advantage over the process by liquid reagents of being applicable to organic fluids. It was accordingly followed in most medico-legal researches until the recent discovery of the methods of Marsh and Reinsch. In order to render it quite satisfactory, it is necessary to go through the steps of the analysis at the same time with distilled water, lest any of the reagents used should accidentally contain arsenic. _Process of Marsh._—This method consists in disengaging arsenic from the solution in the form of arseniuretted-hydrogen gas, burning the gas in such way as to obtain either metallic arsenic or oxide of arsenic, and subjecting the product to various tests. I have called this beautiful method of analysis Marsh’s process, because it appears to me that injustice has been done its discoverer both by himself and those who have since investigated the subject, when they denominated it merely a test. Medico-legal analysis stood in no need of a new test for arsenic, but very much of an easy and infallible method of detaching minute quantities of it in a state of purity from simple and compound fluids, so as to admit of its being accurately examined. It is this important object, and not strictly speaking a new test, that has been attained through means of the discovery of Mr. Marsh.