When the mixture submitted for examination consists of bodies perfectly soluble and perfectly insoluble, simple filtration may suffice to secure their separation; but this is rarely the case, some colloidal material being ordinarily mixed with the crystalloid, and the plan of separating them by _dialysis_, as proposed by Graham, not being altogether successful. When the substances looked for are volatile, distillation may be employed to secure them in a state of purity; in this way prussic acid is separated; but in the case of the poisonous alkaloids other means must be adopted. I. In the separation of such an alkaloid as strychnia, for example, the suspected material is first of all acidulated by one of the weaker mineral or stronger vegetable acids (hydrochloric acid is best), and the whole carefully heated over a water bath. After a time this mixture is to be filtered, and the filtrate well washed with boiling distilled water, and the filtered fluid subjected to evaporation. When dry the substance is to be rubbed up with distilled water, and again filtered; this process to be repeated until a tolerably pure product be obtained. This is next to be neutralized by hydrogen sodium carbonate, or bicarbonate of soda, and the freed alkaloid taken up by rubbing or shaking it with chloroform or ether, and the whole set aside in a well-corked tall test tube. Finally, the ether or chloroform containing the alkaloid is to be removed by a pipette and allowed to evaporate spontaneously, when the alkaloid will probably be left behind in a state fit for testing. This process is a modification of that invented by Stas, in following out the case of the Count Bocarmé. II. For the destruction of organic matter in the search for an inorganic poison, such as arsenic, a process introduced by Fresenius and Von Babo is commonly employed. Its essentials are as follows: The organic matter is to be reduced to as fine shreds as possible, and mixed with about one-eighth of its bulk of pure hydrochloric acid. This is to be heated, and, as it boils, from time to time crystals of potassic chlorate are added, until the solids are reduced to a straw-yellow fluid. This is next treated with hydrogen sodium sulphite, or bisulphate of soda, until a distinct smell of sulphurous acid is given off, after which sulphuretted hydrogen is to be passed through the fluid, concentrated if necessary, for some hours, thus throwing down most metallic poisons in the form of sulphide. This precipitate is to be collected and further tested. When exceedingly small quantities are dealt with the microscope is of use, and the plan of subliming alkaloids and examining their crystals under the microscope, introduced by Guy and Helwig, will be found very useful. The shape of crystalline poisons is a valuable means of determining their identity; arsenic and antimony may thus be readily distinguished, as may other well known substances.