The terms "organic" and "inorganic," as applied to the various kinds of matter of which the universe is composed, had, to begin with, a very definite signification; the latter being applied to all those forms of matter which exist independently of the operation of living beings, whilst all kinds of matter produced by the vital chemistry of living beings were grouped together under the former title. "Inorganic" Chemistry, for example, was that department of chemical science which dealt with the latter class of bodies; while "Organic" Chemistry concerned itself wholly with those of the former group. Even at an early period, however, some confusion was created by the necessity of employing the term "organic" for accumulations of inorganic matter which had at one time entered into the composition of living beings. Thus, limestone is in one sense inorganic, since carbonate of lime, of which it is formed, occurs in nature quite independently of the operation of living beings. In another sense, however, most limestones are organic, since the lime of which they are composed has been in the main derived from the skeletons of animals or plants.

At the present day, the term "organic" has been widely extended in its significance by the wonderful discoveries of modern science; and "Organic Chemistry," as it is still commonly called, embraces a much more extensive field of investigation than would be afforded merely by those substances which are actually manufactured by living beings. In addition, namely, to substances like starch, sugar, fat, and other bodies which are produced solely by the living organism, and which cannot at present be artificially generated, we embrace under the head of "Organic Chemistry" a vast number of compounds which are not produced by living beings, but are artificially manufactured by the chemist in the laboratory. These compounds are derived by various chemical processes from strictly organic substances, which are in reality the product of vital action, and they might therefore be appropriately called " secondary organic bodies."

The link between the primary and secondary organic bodies is afforded by substances such as urea, which is one of the most characteristic of animal products, and which was for a long time unknown except as resulting from animal life. It is now known, however, as first showed by Wohler, that urea is in chemical composition identical with cyanate of ammonia, a substance which can be manufactured on any desired scale in the laboratory. We may reasonably anticipate that the result of more extensive chemical researches will be very largely to increase the number of bodies which, at present recognised exclusively as the products of vital action, will ultimately be found capable of being artificially manufactured.

It need hardly be added, that the term "organic," as applied to any substance, in no way relates to the presence or "absence of life. The materials which compose the living body are, of course, "organic" in the main, but they are equally so after death has occurred - at any rate for a certain time - and some of them continue to be so for an indefinite period after life has departed. Sugar, for example, is an organic product; but in itself it is of course dead, and it retains its stability after the organism which produced it has lost all vitality.

The following are the more important characters which distinguish the various organic substances, whether directly produced by living beings, or secondarily formed by chemical processes of different kinds: (1.) Inorganic bodies are composed of a large number of elements; and these elements are either simple and uncombined, or they are associated into simple compounds, which rarely consist of more than two or three elements united, and are therefore called "binary" or "ternary" compounds. On the other hand, organic bodies are composed of few elements, and these are almost always combined. Indeed there are only four principal organic elements - namely, carbon, hydrogen, oxygen, and nitrogen; and of these the first is so much the most important, that Organic Chemistry has been appropriately termed the "Chemistry of Carbon." Furthermore, the combinations of the elements in organic compounds are complex, the resulting substances being mostly "ternary," "quaternary," or "quinary" compounds; and there is generally a larger number of atoms or equivalents of the combining elements than is usually the case among inorganic bodies. Thus, carbonate of lime consists of no more than one atom of calcium, one of carbon, and three of oxygen. On the other hand, albumen, which may be taken as a typical organic substance, consists of 144 atoms of carbon, no atoms of hydrogen, 18 atoms of nitrogen, 42 atoms of oxygen, and 2 atoms of sulphur. Haemoglobin (the red colouring-matter of the blood), again, is stated by Thudichum to consist of no less than 1875 atoms of no more than six elements. Iron, however, exists in the blood, not improbably in its elemental condition; and copper has been detected in the liver of the mammalia, and largely in the red colouring-matter of the feathers of certain birds, in the latter instance being in a condition of loose chemical combination.

(2.) As the result of the large number of atoms which enter into the composition of organic bodies, we find that substances of this class are singularly unstable - the stability of all chemical combinations, even amongst inorganic bodies, generally decreasing in direct proportion to the increased number of atoms associated in the compound. Organic bodies, being composed of much larger aggregations of atoms than inorganic, are proportionately more unstable; and this instability is increased by the fact that many organic substances contain nitrogen, an element of feeble and undecided affinities, and also by the fact that all those which are of natural and normal occurrence in the living body, are in this state more or less completely permeated with water.

Hence, the primary organic substances, such as enter directly into the composition of living beings, are so unstable that we usually speak of them as decomposing or breaking up "spontaneously," when removed from the influence of the living organism. So long as they form part of the actually living body, they are to some extent stable, but when removed from this they require nothing more than the presence of oxygen, the existence of moisture, and a moderate degree of warmth, to insure their decomposition. These conditions, though essential, are so universally present, that animal and vegetable matters are generally considered as liable to decay "of themselves." If, however, such substances be deprived of access of air, or be frozen, or have their water driven off by desiccation, they are capable of retaining their chemical composition for an apparently indefinite period of time; and one or other of these conditions is carried out in all processes which have as their end the preservation unchanged of the organic substances which form the bodies of animals and plants.