The objects which constitute the vegetable kingdom differ from every species of mineral matter, not only in their peculiar organized structure, but in the chemical arrangement of their elements; those of inorganic matter are generally combined in very simple proportions, as one and one, or one and two, &c. whereas in organized bodies, their proportions are much more complicated; and Dr. Ure observes,[[394]] that such substances derive the peculiar delicacy of their chemical equilibrium, and the consequent facility with which it may be subverted and new modelled, to the multitude of atoms grouped together in a compound; hence too, as Mr. Children[[395]] has observed, is one reason of our utter inability to reproduce even the simplest body of this class, when once its elements have been separated; it is not in the diversity of these elements, but in the manner in which they are grouped, that this peculiarity consists, for, continues the accomplished chemist last mentioned, “vegetable substances seldom contain, as essential, more than three principles—oxygen, hydrogen, and carbon, and sometimes azote. With four simple elements then, a brief alphabet for so comprehensive a history! has a bountiful Omnipotence composed the beautiful volume of the living world, where, turn to what page we may, fresh loveliness meets the eye, fresh cause of admiration and delight.”
The analysis of vegetable bodies resolves itself into two parts, each of which constitutes an equal object of interest to the forensic physician; who, it will be shewn, may occasionally derive important information from both. The first relates to the discovery of the proximate principles of a vegetable substance. The second, to that of its ultimate elements. By the proximate, or, as they are sometimes termed, the immediate principles, we mean those compound substances which exist in the living plant in a state identical with that, under which chemical processes exhibit them, and are chiefly separable by the action of different solvents. The number of these principles is considerable, as gum, starch, sugar, gluten, extractiue, tannin, oils, acid, &c. &c. By the ultimate elements, we understand those, of which the proximate are composed, as oxygen, hydrogen, carbon, and azote. In submitting a plant to destructive analysis, for the purpose of obtaining its ultimate elements, we shall derive compounds, which formed no part of the vegetable structure, and which result from a new arrangement of the elements composing it; acetic and carbonic acids, for example, are obtained by the destructive distillation of several vegetable substances, in which neither of these acids existed ready formed, but only their elements.[[396]] It may easily be imagined to what numerous fallacies such a law of composition must have given origin, in the earlier periods of chemical inquiry; and it is equally evident, that the utmost refinement of chemical science, and the most rigorous methods of analysis, will be required to enable us to deduce any satisfactory conclusion with respect to the quality of a plant, from these data. Such perfection, indeed, has not hitherto been attained, but the period is probably not far distant, when our most sanguine anticipations upon this point may be realised. We have only to trace the history of this branch of chemistry for the last century, to become satisfied of its gradual and important progress towards such an epoch, and of the improvements of which this department of vegetable analysis is farther susceptible; let us, for the sake of illustration, only compare the rude results obtained by the academicians of Paris, at about the commencement of the seventeenth century, with those of MM. Gay-Lussac and Thenard[[397]], or with those, very lately instituted in this country by Dr. Ure,[[398]] and we shall perceive that while the former of these experimentalists, by the aid of heat, were unable to form the slightest distinction between the most inert, and the most poisonous species of plants, the latter, by means of the same agent, aided by the modern doctrine of equivalent ratios, has succeeded in establishing the proportions in which the elements of each vegetable body combines; and with such accuracy, as to discriminate between substances, which bear the greatest analogy to each other; as between the varieties of sugar, and those of oil; and even between common flax, and the same substance prepared according to the improved process of Mr. Lee. This statement is sufficient to show the capability of ultimate analysis, on certain occasions, to identify vegetable bodies; but we are, at present, scarcely advanced far enough in such an investigation, to make it subservient to the detection of vegetable poisons. Nor has our knowledge with regard to proximate analysis, been less successfully advanced. The late researches of the French and German chemists have demonstrated the existence of several new alkaline bodies in the class of vegetable poisons, to which some of these plants appear to be exclusively indebted for their activity, as the poppy, hellebore, colchicum, &c.; and whose characters are so distinct and striking, as to enable the chemist to recognise their presence by appropriate re-agents. In other cases, the virulence of a plant would appear to depend upon the combination of several[[399]] proximate principles; while in some few instances there exist in the same individual vegetable, two distinct elements of activity, as illustrated by the interesting history of tobacco.
In cases of vegetable poisoning it will occasionally occur, that some remains of the plant may be collected; and seeds, portions of the fungi, and leaves, may be found in the contents of the stomach; whence a knowledge of botany becomes indispensable. This branch of science is, moreover, important to the toxicologist, as enabling him to pursue the study of plants with greater precision; for experience has shewn that there is a wonderful analogy between the structure of those plants which resemble each other in medicinal operation. Thus those which, from their dismal and dusky appearance, have been arranged under the title of Luridæ, are in general highly poisonous; they also possess a very peculiar and disagreeable smell, so that Nature has, upon this occasion, kindly given us notice of approaching danger, by means of our senses.
Of equal importance with the knowledge of the generic and specific characters, is that of their sensible qualities, and the changes which these latter undergo by pharmaceutical preparation.
Cl. III. ACRID, or RUBEFACIENT POISONS.
Most of the subjects of this class constitute articles of Materia Medica; so that ignorance on the one hand, and accident on the other, may render them the unexpected source of mischief. With respect to the physiological action of these bodies, the reader has only to refer to our classification at page [207], to perceive that it will not admit of generalization; for each division, it will be observed, contains individuals which belong to the class of acrid poisons.
As the history of most of these articles is to be found in works on Materia Medica, we shall not enter so fully into their properties, as we might otherwise have considered necessary.
Camboge or Gamboge.
This beautiful gum-resin is obtained by making incisions in the leaves and young sprouts of the Stalagmitis Cambogioides[[400]] (Polygamia Monæcia—Nat. ord. Tricoccæ. Wild:) It is first collected, in the kingdoms of Siam and Ceylon, in cocoa-nut shells, and is thence transferred into large earthen jars, where it remains until it is nearly dried to a cake, when it is formed into rolls, and wrapped up in leaves. It is imported into Europe[[401]] in cases and boxes. Its deep yellow colour, which is so materially brightened by being wetted, and its shining fracture, are characters sufficiently striking to enable the practitioner to identify it; and when we add to these the history of its habitudes with different menstrua, the chemist will have no difficulty in detecting its presence, viz. when triturated with water, two-thirds of its substance are speedily dissolved, and a turbid solution results; alcohol dissolves nine-tenths, and forms a yellow transparent tincture, which is rendered turbid by the addition of water; sulphuric ether dissolves six-tenths of the substance; it is also soluble in alkaline solutions, and the resulting compound is not rendered turbid by water, but is instantly decomposed by acids, and the precipitate so produced is of an extremely brilliant yellow colour, and soluble in an excess of acid.
Its action upon the animal œconomy is that of a powerfully drastic purge. We are, however, not acquainted with any case in which death followed its administration. From the experiments made upon animals, it would appear to produce its effects by a local action on the textures, with which it comes in contact, and it will accordingly be found in the third class of our physiological classification, (page [207].)