V. CHANGES IN COMPOSITION.—The ali mentary materials which serve as the food of the living organism, cannot be appropriated by its several tissues, and rendered like themselves in structure and properties, until they have un dergone certain changes in composition, by which the proximate principles are produced. It is by the organisation of these compounds, that the constant disintegration of the elemen tary parts of the living system is compensated, and those vital properties maintained, the exer cise of which forms an essential part of the circle of actions involved in life. Another class of changes in composition consists in the production, from the same materials, of the peculiar ingredients which characterise each se creted product; some of these may be regarded as directly eliminated from the nutritious gredients of the blood, in the same manner as are the solid tissues themselves; whilst others would rather seem to result from the new com bination of the disintegrated elements, which are taken up and removed by the current of the circulation, and carried to organs destined to separate them entirely from the living portions of the system. All these changes are frequently said to be effected by a vital chemistry ; or (to speak in more precise language) to result from the operation of vital affinities, of a different character from those ordinary chemical affinities which produce the well-known changes in the inorganic world. In conformity with the New tonian direction to avoid unnecessarily multi plying causes, we shall briefly examine the grounds upon which this hypothesis is based, and enquire whether it is requisite for the ex planation of phenomena, or even gives us any assistance in our researches.
The chief ground for the assumption of a distinct set of vital affinities appears to be, that the mode of union of the elements of the organic compounds is essentially different from that which prevails in the inorganic world ; and that the chemist, who has the power of effecting or controlling those changes which are produced by physical laws, and can therefore imitate to a great extent the immense variety of combinations which the mineral kingdom af fords, is unable to effect or control the action of similar materials, so as to produce any of the class of organic compounds or proximate prin ciples. It has, until very recently, been re garded as a distinctive character of organic compounds, that their elements are combined in ternary or quaternary arrangements of com plex nature, in which each ingredient is equally united with all the rest; whilst all inorganic substances admit of being ultimately resolved into simple binary combinations. Thusbrin is regarded as composed of 6 parts of carbon, 2 of oxygen, 5 of hydrogen, and 1 of nitrogen ; and these elements are imagined to form a qua ternary compound, all having a mutual attraction for each other ; whilst carbonate- of ammonia, which consists of 1 carbon, 2 oxygen, 3 hy drogen, and 1 nitrogen, is a binary combination of two other binary compounds, carbonic acid and ammonia. Berm this it may be remarked, that there are undoubtedly some proximate principles, (that is to say, the simplest forms to which organic compounds can be reduced, without altogether disuniting them into their ultimate elements,) which consist of two ele ments alone, and which exist in this simple form in living bodies. Such are some of the compounds of carbon and hydrogen. Further, the rapid progress of analytic research is leading to the belief that the complex arrangements just referred to may be resolved into those of a binary character; so that most organic com pounds may be regarded as resulting from the union of others of simpler nature, just as a salt is formed by the union of an acid and an alkali. The discovery of cyanogen, and of its capabi lity of acting as a compound radical,—uniting, like chlorine or iodine, with hydrogen to form an acid, and even occasionally like oxygen or sulphur, in combination with some metals, as the base or alkali to such an acid,- was the first step in a career of brilliant disco veries, which, even at the present day, may be regarded as scarcely commenced. When cy
anogen combines with a metal, the combination is in reality a ternary one, although in all its . properties it has a binary character. Thus, the cyanuret of silver (whose ultimate composition is 1 part of the metal, with 2 carbon, and 1 nitrogen,) will form a salt, in which it acts as the acid or negative ingredient, with the cya nuret of potassium ; and the soluble cyanurets will form salts with the chlorides or iodides of the metals, thus establishing their claim to a binary character. But still further;—cyanogen in combination with iron appears itself to act as a compound radical, combining as a simple body with other elementary substances.* From the analogy afforded by this and other in stances, many chemists are now disposed to look upon the combination of the oxy-salts in a new light. It is suspected that, when sul phuric acid and soda are brought together, the resulting compound is not formed by the union of an atom of the acid with an atom of the alkali, but by the generation of a new com pound radical, sulphato.xygcss, consisting of I ' part of sulphur with 4 of oxygen, which unites as a simple body, like chlorine, iodine, or cya nogen, with the metal sodium.* It will be seen, then, that the tendency of modern researches in inorganic chemistry is to prove, that the mode of combination which characterises the union of its elements, is not by any means so simple as it has been usually supposed ; but that a binary, a ternary, and perhaps even a quaternary compound, may perform the part of an element, combining with other elements which are really simple, to form what are regarded as simple binary com pounds. We shall now enquire what reason there is for believing that the compounds with which organic chemistry supplies us, have a similar constitution. We must be content, however, with selecting one or two examples from among the vast number which the in dustry of analytic chemists is constantly bring ing to light. The vegetable alkaloids have been generally regarded as proximate principles, not to be separated into simple compounds without an entire disunion of their elements. They all contain one equivalent of nitrogen ; and there is good reason to suspect that this element is not equally combined with all the rest, but exists in union with hydrogen in the form of ammonia, to which the alkaline power of these substances is due. Again, camphor was long considered a proximate principle of ternary composition ; but it is now found to be an oxide of cuwxphene,—a compound radical con sisting of carbon and hydrogen, which will unite, like cyanogen, with simple bodies ; form ing camphoric acid with another equivalent of oxygen, and entering with chlorine, &c. into other compounds. Lastly, urea may be men tioned, in which the four elements that com pose it may be regarded as existing in several forms of binary combination. It contains these elements in the following proportions :-2 oxy gen, 4 hydrogen, 2 carbon, and 2 nitrogen. These may be considered as existing in the form of cyanic acid, ammonia, and water ; one equivalent of each of these forming cyanate of ammonia; and, in fact, by the artificial union of these compounds, urea has actually been produced. It is by no means certain, however, that these compounds exist as such in urea ; and various ideas of its composition are enter tained by chemists, on which this is not the place to comment. Our object is simply to show the analogy in the composition of the products of vital chemistry with that of the ar tificial compounds whose formation is subject to none but physical laws. Why the chemist is not more successful in imitating in his labo ratory the operations of the living economy, will presently become subject for consideration.