After a comparative trial of the most esteemed process, Drs. Fresenius and von Babo state that they found the one now described as delicate as any other, and the only method by which the quantity of oxide of arsenic can be ascertained with accuracy.—The hydrochloric acid used at the commencement enables the water to dissolve compounds of arsenic which water alone will not act on; and it farther facilitates solution by breaking up or dissolving organic textures. The addition of chlorate of potash prevents the escape of oxide of arsenic during the subsequent evaporation; which is apt to happen when hydrochloric acid is present. The subsequent addition of sulphuric acid converts arsenic acid into arsenious acid, in which shape the sulphuret of arsenic is more readily formed by the action of hydrosulphuric acid gas, when organic matter co-exists in the solution. The steps for destroying organic matter thrown down with the sulphuret at its first formation require no further commentary: They are the most important particulars in the process for its main object,—the determination of the quantity of pure sesqui-sulphuret, and, through it, of the sesquioxide originally in the subject of analysis. _Of certain alleged Fallacies in the case of Organic Mixtures._ Before taking leave of the detection of arsenic in organic mixtures, it is necessary to notice certain alleged fallacies in the way of every process, arising from arsenic obtaining admission into the subject of analysis through other means than its intentional addition or its introduction as a poison into the body. This topic, one of paramount importance in medico-legal chemistry, has lately undergone careful investigation during and since the notorious trial of Madame Lafarge. The results are the following:— It has been alleged that arsenic may obtain accidental admission into the subject of analysis, 1, because the reagents used in the processes may be adulterated with arsenic; 2, because the material of the apparatus may contain it; 3, because it may have existed in antidotes administered during life; 4, because it sometimes forms a constituent part of the human body in the natural state; and 5, because it exists in the soil of some churchyards.