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symbol is one with the aid of which alone, perhaps, the tion derivatives are formed by the displacement of the general behaviour of naphthalene may be satisfactorily ex- hydrogen atoms in the median ring. The following place, it harmonizes formulae afford a satisfactory expression of these facts, pressed. In the first two rings are in corn- it being understood that the hexagons marked C have the with the fact that the one responding to centric structure :— plete sympathy, the the other. Then, on every change made in tively slight — beaccount of the relaC 'C_ _/C ence which would be cause divided—influ_/ two rings by the exercised upon the An entire change of properties is produced by saturating two affinities common to both, the remaining four centric the two carbon atoms of the median ring in anthracene, affinities of each ring would presumably be less attracted and there can be little doubt that the lateral ethenoid into the ring than in the case of benzene; consequently ring lapses into the centric state, a symmetrical compound they would be more active outwards, and combination being produced. Thus anthracene dihydride has more of would set in more readily. When, as in the formation of the properties of a saturated hydrocarbon, and anthranaphthalene tetrachloride, for example, the one ring beO comes saturated, the other might be expected to assume the normal centric form and become relatively inactive. quinone is characterized by extreme inThis is absolutely the case. On the other hand, if substitution be effected in the one ring, and the affinities in that ring become attracted inwards, as apparently happens in the case of benzene, the adjoining ring should become ertness; moreover, when the latter is attacked both relatively more active because the common affinities would lateral rings are equally readily acted upon. On the act less into it. Hence, unless the radicle introduced be other hand, the behaviour of phenanthraquinone in no one which exercises a special attractive influence, substitu- way differs in principle from that of phenanthrene. It is tion should take place in preference in the previously noteworthy that although the lateral rings in phenanunsubstituted ring. In practice this usually occurs; threne are in the condition of ortho-di-derivatives of for example, on further bromination, a - bromonaphtha- benzene, they do not display the activity of ortho-homolene yields a mixture of the two dibromonaphthalenes logues of benzene—which is a fact not without bearing on the question as to the character of the influence exercised Br Br by hydrocarbon radicles in promoting substitution. It has been previously pointed out that the evidence favours A v0 and > an<l when nitronaphthalene is either the view that the open chain hydrocarbon radicle in a Br Br benzenoid hydrocarbon has an attractive influence combrominated, or nitrated or sulphonated, the action is practi- parable with that exercised by oxygen in phenols and cally confined to the second ring. An alternative centric by nitrogen in amines; as it is not to be expected that formula for naphthalene is that proposed by Bamberger. the median closed chain which in phenanthrene occupies Instead of representing it as a dicyclic hydro- the place of the two separate radicles in an ortho-dicarbon, formed by fusion of two benzene rings, derivative of benzene would have any such influence, the this indicates that it is a monocycloid composed behaviour of phenanthrene lends considerable support to of ten atoms of carbon. The formula has the this view. The argument may be one on which no great advantage that it may be constructed from stress should be laid ; it is advanced mainly with the tetrahedral models of the carbon atom ; but it involves the object of calling attention to the way in which the study assumption that the molecule has within it a mechanism, of polycyclic hydrocarbons may serve to elucidate quesequivalent in a measure to a system of railway points, which tions of general importance. The bearing of individual can readily close up and pass into that characteristic of facts on the broad problems of chemistry is too commonly overlooked nowadays, and it is very necessary that chemists benzene. The isomeric hydrocarbons anthracene and phen- should seek to appreciate more fully than they do at anthrene, C14H10, are to be regarded as formed by the present the general interdependence of chemical phenofusion of three benzene rings in the manner represented mena, and to correlate their observations more carefully with the existing body of facts. by the symbols It remains to consider the behaviour of mixed cycloids in which multivalent elements other than carbon are 1 O ^ included; this is in many respects very remark- Hetero. Phenanthrene. Anthracene. able. Pyridine and thiophene may be taken as cycloids. Their behaviour, and therefore their inner structure, is examples. Pyridine, C5NH5, is derived from very different, however, and particularly noteworthy in benzene by the displacement of one of the CH groups by comparison with that of naphthalene. In both cases the an atom of nitrogen. The effect of this change is to median ring is most easily acted on; but whereas phen- lower materially the activity of the cycloid in comparison anthrene behaves as a symmetrical compound, the behaviour with that of benzene. Although the nitrogen in pyridine of anthracene is that of an unsymmetrical compound, one exhibits marked basic properties, apparently it has little of its lateral rings being far more readily attacked than or no influence—such as is exercised by the nitrogen in the other. For example, both hydrocarbons at once aniline—in promoting the introduction of substituting combine with bromine, forming unstable dibromides, which groups into the ring in place of hydrogen, substitution are readily resolved into hydrogen bromide and mono- being effected only with difficulty. The arguments which bromo - substitution derivatives; the further action of favour the adoption of the centric formula for benzene bromine leads to the production of a di-substitution- apply with even greater force to pyridine, and render such derivative. The action then stops in the case of a formula for this compound almost a necessity, but the phenanthrene, unless forced, but dibrom anthracene adoption of the centric formula for pyridine has important readily combines with bromine, forming dibromanthra- consequences, as it involves the conclusion that the addicene tetrabromide. In both cases the dibromo-substitu- tion of a nitrogen affinity to five carbon affinities not only