ORGANIC RADICALS. The large number of simple atoms which generally go to form an organic substance are not bound together into one homogeneous compound, but are to a great extent arranged in groups, each group playing the part of a simple or elementary body. Such groups performing this function are termed compound radicals. Thus, alcohol contains 4 equivalents of carbon, ti of hydrogen, and 2 of oxygen, but these elements are supposed to be arranged in the alcohol in the following manner 1 0 That is, 4 equivalents of carbon and 5 of hydrogen exist in the alcohol in the form of a compound as distinct and independent as the remaining equivalent of hydrogen ; this compound of carbon and hydrogen, which is called ethyl, is an example of an organic radical, or of an organic group playing the part of an elementary body : for by certain chemical operations, the radical ethyl can be removed from alcohol and its place supplied by hydrogen, a kind of substitution which transforms the alcohol into water. Thus alcohol and water are related to each other, as shown in the following formulte :— COI, 1 0 1 11 J 102 Alcohol. Water.
The simple or elementary radicals comprise, on the one hand, bodies which, like potassium, zinc, iron, and hydrogen, are electropositive or basylous in their character ; and, on the other, substances such as oxygen, sulphur, and chlorine, which are electronegative or ailerons in their behaviour. The organic radicals admit of a precisely similar classification : some of them, such as methyl, ethyl, phenyl, &c., imitating hydrogen, zinc, or iron in their functions ; whilst formyl, ethyl, cyanogen, fie., resemble chlorine and sulphur in their chemical phenomenon which is strikingly seen in the case of highly polyatomic metals, such as arsenic and antimony. Thus tribasic arsenic acid, by the substitution of an equivalent of methyl for oxygen, yields the bibasic monomethyl-arsenic acid, a well defined acid of considerable energy, though inferior in chlorous power to arsenic acid. The like substitution of a second equivalent of methyl for oxygen reduces the chlorous character of the body to the comparatively feeble condition in which we find it in cacodylic acid, which is incapable of forming an ammonia salt. A similar substitution for the third time overpowers the chlorous character of the compound altogether, and we now have a feeble biacid base—the arsenic dioxymethide ; which again, by the exchange of a fourth atom of oxygen for methyl, is transformed into the oxide of tetramethylarsonium, a base of such energy as to be comparable with the caustic alkalies themselves.
The history of the organometallic bodies teaches forcibly a doctrine which affects all chemical compounds, and which may be called the doctrine of atomic saturation ; each element is capable of combining with a certain limited number of atoms, and this number can never be exceeded, although the energy of its affinities may have been increased by combination up- to this point. Thus zinc attains its atomic satura
tion by uniting with only one atom of another body, in other words, it is uniatomic ; consequently, the zinc compounds of the alcohol radicals, notwithstanding their intense affinities, are incapable of com bination. The action of chlorous elements upon them is one of substitution, not of combination. Polyatomic metals exhibit the same phenomenon : a double atom of tin cannot combine with more than 4 atoms ; a single atom of arsenic or antimony with more than 5 atoms of other bodies. But in the combinations of polyatomic metals, we frequently notice from the lowest to the saturated compound one or more intermediate points of exalted stability ; thus, antimony has a teratomic stage of comparative stability ; nitrogen, phosphorus, and arsenic, whilst exhibiting a similar teratomic stage, have also a biatomio one, though of greatly inferior stability ; whilst the existence of pro toxide of nitrogen, and especially the recent researches of Mr. Griess render it more than probable that nitrogen has a third and uniatomic stage. In bodies possessing at least one stage of stability below saturation, and in which all the atoms united with the polyatomic element are of the same kind, the stage of maximum stability is very rarely that of saturation. Thus in nitrogen, arsenic, and bismuth compounds of the kind just mentioned, the stage of maximum stability is decidedly the teratomic one ; in antimonial compounds of a similar nature, the teratomic is also, though less decidedly, the stage of maximum stability ; whilst in phosphorous compounds, the points of maximum stability and saturation generally coincide. When, however, the atoms united with the polyatomic element are not of the same kind, then the stage of maximum stability usually coincides with that of saturation. Thus the binoxido or bichloride of triethylarsine, or triethylatibine, are more stable than triethylarsine or triethylstibine themselves ; but this pentatonic stability reaches its climax in arsonium, stibonium, and phosphouium compounds, as it does also in the corres ponding compounds of nitrogen, although the latter element exhibits a much stronger tendency towards universal teratomic stability than its chemical associates.