The detection of a poison is, in many instances, no easy matter; it should not therefore be rashly undertaken, except by one well skilled in the minutiÆ of the processes to be adopted; but on the other hand, there are so many points of practical importance which may be noted by the practitioner in charge, that his attention should be specially directed to these. The exact determination of the cause of death will depend partly on the symptoms noted during life and on the appearances found after death. These come within the province of the ordinary practitioner; on the other hand, the special physical, chemical and physiological portion of the inquiry should be referred to the expert. In a book of this scope it is impossible to give full details with regard to these last, but a brief sketch nay be useful.

The physical examination, say of the contents of the stomach and intestines, should commence with noting the smell, color, and general appearance of the matters submitted for examination. The odor, for instance, may be useful in indicating the presence of prussic acid, of alcohol, of opium, or of phosphorus. The color may indicate the presence of salts of copper, of fragments of cantharides, &c. The general appearance may give some clue to the mode of introducing the poison, the kind of food or drink used to conceal it, &c. Seeds of poisonous plants may be found: this is especially the case in India, where the seeds of datura are frequently used for criminal purposes; or the poisonous substance may have been administered in such quantity that a portion of it may at once be secured for analysis. This not unfrequently happens in poisoning by arsenic. It is not enough to employ the naked eye in examining these suspected matters; a hand lens of some power should be used; in this way characteristic crystalline forms, botanical peculiarities, and such like, may be made out.

Still these are merely introductory to the most important part of the research, which ought to be undertaken systematically, especially if the quantity of material to be operated on is small. Most frequently the matters to be examined are mixed, some soluble, some insoluble; but there may also be submitted for analysis portions of the pure substance. The object of the analyst is to obtain the substances which he has to examine chemically in as pure a condition as possible, so that there may be no doubt about the results of his testing; also, of course, to separate active substances from those that are inert, all being mixed together in the stomach and alimentary canal. Again, in dealing with such fluids as the blood, or the tissues of the body, their natural constituents must be got rid of before the foreign and poisonous body can be reached. There is this difficulty further to contend with: that some of the most poisonous of substances are of unstable composition and readily altered by chemical reagents; to this group belong many vegetable and most animal poisons. These, therefore, must be treated differently from the more stable inorganic compounds. With an inorganic poison we may destroy all organic materials mixed with it, trusting to find it still recognizable after the process; not so with an organic substance: that must be separated by other than destructive means.

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.

The spectrum has not yet been applied to toxicological research, although it has been employed to determine the existence of a blood stain.