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§ I. Analysis of Organic Compounds;—§ II. Principles of Classification;-§ III. Metamorphoses of Organic Compounds.

(1035) ORGANIC CHEMISTRY, in its primary signification, is that division of the science which treats of the chemistry of organized beings, and of the products of their chemical actions upon matter in its various forms.

It is important in limine to draw a clear distinction between organic compounds and organized bodies.

Organic compounds, like those derived from the mineral kingdom, possess a definite composition, and in many cases exhibit a perfectly definite crystalline structure. Bodies, such as sugar, oxalic acid, urea, taurin, and quinia, belong to this class; such substances are often spoken of as the proximate principles of animals and vegetables: many of them may be obtained from inorganic bodies by synthesis.

On the other hand, organized bodies, such as muscular tissue, nervous structure, cellulin, and ligneous fibre, never exhibit any tendency to crystalline arrangement, but show a rounded, vesicular, or fibrous configuration; and are so connected with each other as to form parts of a system, each of which is incomplete if severed from the remainder. Bodies such as these cannot be prepared synthetically: they are the result of the action of living bodies upon inanimate matter; a living body having the power of assimilating fresh particles, and of arranging them in the special form which characterizes the class to which the individual organism belongs.

The study of the chemical changes that occur during these transformations constitutes physiological chemistry, the most diffi

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gult branch of the science. Its difficulty depends, however, not simply upon the obscurity which enshrouds the nature of life itself; for the essential nature of every description of force, and the mysterious tie which exists between matter and force, has baffled the penetration of the profoundest philosophers, and belongs to an order of truths to which the human intellect probably may not be permitted in this sphere of its existence to attain.

The difficulty in the study of the chemical changes that occur in the living animal or plant depends rather upon the extreme delicacy of the arrangements by which these metamorphoses are effected; and upon the complicated nature of those arrangements, which are liable to injury from a multiplicity of causes that have hitherto eluded scrutiny. Other difficulties arise from the facility with which important changes are effected in the organism itself without being appreciable to human sense, save in their results, although they may even proceed so far as to occasion death. The ligature of a large artery or the division of a nerve effects an alteration which can be at once appreciated; but when an animal is poisoned by a fraction of a grain of aconitina or by a few drops of hydrocyanic acid, there is nothing to indicate to the anatomist, to the microscopic observer, or to the chemist, the mode in which these bodies operate in suddenly cutting short the life of the animal. The organization still appears to be perfect, yet the living laboratory no longer continues to perform its wonted operations, and the inanimate mass speedily falls under the dominion of ordinary chemical laws. In the living body, ordinary chemical actions are suspended, and compounds are produced, which, when released from the influences under which they originated, quickly undergo fresh changes, which are manifested by the occurrence of putrefaction and decay, to which both animal and vegetable bodies are liable when they no longer form parts of the living frame. Occasional instances of instability, somewhat analogous to those exhibited by organized bodies, are seen in some of the compounds of inorganic chemistry; but this perpetual tendency to change is one of the distinguishing and essential characteristics of living bodies. When the plant or the animal is performing its functions in a healthy manner, these changes succeed each other in a defined and regular order; but if this sequence be modified, even in a slight degree, disease generally ensues, and when the alteration reaches a certain amount or degree, death is the speedy and inevitable consequence.

In the present work the attention of the reader will be only

CONSTITUENTS OF ORGANIC PRODUCTS.

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incidentally directed to the physiological portion of the subject, the main object being to exhibit the mutual relations which the different chemical compounds bear to each other, and to trace the laws of their formation. Even with these limitations the subject of organic chemistry is one of great extent and complexity.

(1036) Organic products are composed of a small number of elements: yet such products present properties as much opposed to each other as it is possible to imagine: some constituting the daily food of man, others acting as direct and violent poisons: yet these very different bodies rarely consist of more than four elements, viz., carbon, hydrogen, nitrogen, and oxygen; a few contain minute proportions of sulphur and phosphorus, with certain earthy and saline matters.

Carbon is the characteristic element in organic bodies, so that generally the most convenient means of ascertaining if an unknown body be of organic origin, is to heat a fragment of it in a narrow test tube; the carbon is thus separated, and the substance becomes charred and blackened, except in a few cases where the material may be wholly volatilized without undergoing decomposition. Several of the essential oils, such as those of turpentine, elemi, juniper, and attar of roses, consist of carbon and hydrogen only the same may be said of naphthalin, benzol, and the illuminating constituents of coal gas; but the larger number of organic bodies contain carbon, hydrogen, and oxygen; this occurs, for example, in the case of sugar, acetic acid, alcohol, the fixed oils, and the greater part of the proximate principles of vegetables. Nitrogen is superadded to these elements in many instances, though such compounds are of less frequent occurrence, except in highly organized portions of the plant or animal, such as the seed, or the muscular and other animal tissues; albumen, fibrin, indigo, quinia, and the vegetable alkalies generally, contain nitrogen as an essential component. The presence of nitrogen may in most cases be ascertained readily by heating a small fragment of the substance in a tube with solid caustic potash, when a distinct odour of ammonia is perceived.

The organs by which these various compounds are produced always contain small quantities of salts of the alkalies and earths, among which the compounds of potassium, sodium, calcium, and magnesium, in the form of chlorides, sulphates, phosphates, and silicates, are the most frequent in rarer cases salts of iron and manganese are met with, and in still rarer instances, fluorides, iodides, and bromides. The presence of certain of these saline. bodies appears to be a condition as essential to the chemical

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