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.
There is nothing more difficult in toxicology than to give a satisfactory classification of poisons, insomuch that some have fallen back on the no-classification, or natural history system, and grouped poisons as mineral, vegetable, and animal, according to their source. In despair of achieving anything better, a modification of the old and well-known system is here followed, poisons being classed as Corrosives, Irritants (Simple and Specific), and Neurotics; the latter group is, however, further subdivided, something in the same way as that adopted by Dr. Guy. The group of corrosives should comprehend all which by contact destroy the bodily textures, and so occasion death. These same substances, when diluted, may be incapable of destroying the tissues directly, but may do so by setting up inflammation; these with certain others having like effects, would form the group of simple irritants. They kill by virtue of their secondary effects on the constitution. But some substances, like arsenic, are not only capable of inducing local inflammations, with their secondary effects, but are also possessed of certain specific and well marked properties differing in each case. These are _specific irritants_.
Neurotics comprehend all poisons whose effects are mostly referable to the nervous system, necessarily a most diverse group, which we are not yet in a position to minutely analyze. Some, however, act mainly on the brain, some on the spinal cord, some on certain nerves only, or on the vasomotor system of nerves; some act it is hardly possible to tell how.
There was an old group of _septic_ poisons; to this might still be referred certain noxious gases, such as sulphuretted hydrogen; or were it made to include all poisons acting directly on the blood, it would include the still more dangerous gas, carbonic oxide.