This group of acids, which is one of the most complete of the homologous series, illustrates in a very interesting manner the general similarity in properties which always exists in homologous bodies. This statement may appear paradoxical when the extreme terms of the series are compared together, since formic acid is an exceedingly pungent, corrosive, volatile liquid, which requires cooling below 32°, in order to render it solid, and it is miscible with water in all proportions; whilst melissic acid requires a temperature of 192° (89° C.) for its fusion, is insoluble in water, and has the properties of a fat: but if we compare together the contiguous terms of the series, such as the formic with the acetic, the acetic with the propionic acid, and so on, no abrupt change in properties will be perceived between the contiguous terms; each, as the proportion of carbon and hydrogen increases, becomes gradually less and less volatile, less fusible, less susceptible of solution in water, until we reach the true fatty acids. Now this similarity in properties, which is observed equally in other groups of homologous substances, naturally leads to the supposition that there must be a similarity in chemical constitution. It is not unreasonable to suppose that the acids HH-10, are all formed upon the plan or type of the simplest of them, viz., formic acid. Various theories may be advanced as to the composition of formic acid, but whatever that composition may be, we may imagine with Wurtz and with Gerhardt that the other acids are so many varieties of formic acid, which contain ethyl, trityl, tetryl, &c., instead of hydrogen; for instance : Formic acid n Acetic, or methylformic 2 a somewhat similar hypothesis might be extended to the different alcohols themselves. It will be perceived that all these groups, derived from the alcohols by decomposition, are produced by an analogous decomposition of the several alcohols, and that they all preserve a strict homology in their respective groups. This will be more easily traced by the aid of the table in the next page, in which the different classes of compounds just described are so arranged that all those compounds which are placed in the same vertical column are homologous, whilst those which are placed in different columns are heterologous, or dissimilar in molecular constitution and in chemi 40 CLASSIFICATION IN HOMOLOGOUS AND COLLATERAL SERIES. cal characters: but it will be seen that those bodies in the same horizontal line are all related to the same alcohol; they are all formed from it by reactions similar to those by which each of the corresponding compounds in the other horizontal lines are formed from their alcohol. Exceptions to this uniformity occur in the last column in the case of the compound ethers, the formula for which, in the majority of instances, represents the ether formed by the union of the acid with vinic ether, since these compounds have been more generally formed and studied than any other kind of compound ether; though in a few instances the compound ether resulting from the union of the acid of the series with its corresponding ether, is given, and there can be no doubt that such ethers CLASSIFICATION IN HOMOLOGOUS AND COLLATERAL SERIES. 41 might in all cases be formed if any sufficient inducement to their preparation existed; or if the materials required were sufficiently abundant. The column headed mercaptans contains a series of compounds which correspond to the alcohols, and are strictly analogous to them; they contain sulphur in the place of oxygen; the sulphides are analogous in like manner to the ethers; and there are bromides and iodides, which are not included in the table, analogous to the chlorides. The quantities represented by the formulæ given in the table yield, in all cases where the compound is volatile without decomposition, 2 volumes of vapour if H=1 be taken as I vol. In this table many blanks occur, which could in most cases be filled up if the compounds which correspond to 42 THEORY OF COMPOUND RADICLES. them presented sufficient interest to induce the chemist to prepare them. New compounds are indeed being continually added to the list. By thus mapping out the territory which has been explored, the progress of future research is materially aided, and each new compound, as it is discovered, more readily falls into its true place, while its relations to other bodies previously known are anticipated, and readily verified. Exact observations upon the crystalline form of homologous bodies are still wanted; but it appears that in many cases homologous compounds are isomorphous, though this is not by any means uniformly true. The barytic methyl-sulphate and ethylsulphate appear to be isomorphous; such is the case also with the alums formed with aluminic sulphate by the sulphates of the alcohol bases, methylia, ethylia, amylia, and trimethylia; but the double salts formed by the hydrochlorates of these bases with platinic chloride are decidedly not isomorphous. The double salt of hydrochlorate of trimethylia and platinic chloride is isomorphous with the corresponding salt of ammonia; but the corresponding double salt of ethylia crystallizes in rhombohedra, and that of diethylia in oblique prisms. (1060) Theory of Compound Radicles.-Liebig explained the similarity in properties between the terms of homologous series by supposing the existence in each, of a certain group of elements which he regarded as the radicle of the series. He, in fact, made the theory of compound radicles,* as it is termed, the basis of his classification, and defined organic chemistry to be the chemistry of compound radicles. According to this supposition, the basis of each ether is a hydrocarbon, from which all the heterologous bodies of the series, or bodies not analogous in composition, are derived for example, Upon this theory the simple ethers are oxides of the peculiar electro *The German term radikal (from the Latin radix, a root) is commonly, but inaccurately, translated radical, which is properly an adjective, the word radicle being the appropriate rendering. THEORY OF COMPOUND RADICLES. 43 positive hydrocarbon which forms the radicle of the series; the alcohols are the hydrated oxides of the same radicle, and the socalled hydrochloric, hydrobromic, hydriodic, and hydrosulphuric ethers, are compounds in which chlorine, bromine, iodine, or sulphur have united with the radicle-just as in the corresponding compounds of a metal; whilst the compound group termed ethyl (Є,H,) discharges a function in these compounds analogous to that of potassium in its salts: for example, calling ethyl (€2H¿)=Et, the two series would run as follows: The mercaptans correspond in composition to alcohol in which the place of the oxygen is supplied by sulphur; whilst the compound ethers, such as the formic, the acetic, and the valeric, are regarded in the light of salts, in which the hydrogen of the formic, the acetic, or valeric acid is displaced by methyl, ethyl, or some other compound radicle, by which its acid properties are completely neutralized. It is true that the theory of compound radicles supposes the existence of a number of substances which have not been isolated, and also of many which probably have no separate existence. The theory indeed cannot be supposed to afford a correct representation of the molecular constitution of organic bodies; but it is a convenient fiction, which materially facilitates the retention of their composition in the memory, and the comprehension of the metamorphoses which such compounds experience. One of the first groups to which it was systematically applied by Liebig and Wöhler was that derived from the essential oil of bitter almonds. This oil was regarded by these chemists as containing an organic radicle, to which they gave the name of benzoyl (Є,H.), and which may be conveniently distinguished by the symbol Bz. Although unsuccessful in the attempt to isolate this radicle, they succeeded in transferring the group of which it was supposed to consist from one elementary substance to another, and in causing it to enter into combination with other organic groups; and by the aid of this hypothesis they were enabled to give as simple and intelligible an account of the mode in which the compounds derived from the oil were formed, as though they had been dealing with the salts or other compounds of a metal. The advantages thus ob |