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all-important part, not merely in facilitating the occurrence radicles, into which the agent is resolved, become disof change, but also in determining its course. Probably tributed and combine with the ethenoid carbon atoms. it acts in more ways than one, both determining the initial Such changes occur, as a rule, very readily ; indeed, there is attack by coupling halogen with hydride and also pro- reason to suppose that the combining power of ethenoids moting the separation of halhydride (hydrogen bromide has been considerably underrated. Their activity must or chloride). If it be assumed that the catalyst combines be ascribed to the “ attractiveness ” of the two contiguous either with the halogen atom or with the carbon atom carbon atoms united by ethenoid affinities, a form of union with which this latter is associated, it would in a measure which leaves these atoms endowed with considerable protect the hydrogen also associated with the carbon residual affinity. Whatever the nature of this form of atom, and project the attack, as it were, upon the hydrogen union may be, it has clearly less than twice the force of atom associated with the contiguous carbon atom. In that by which carbon atoms are held together in paraffinsuch cases it may well happen that an ethenoid compound oid compounds ; consequently the use of the terms, “ united is either potentially or actually formed as an intermediate by double bonds ” and “ by two affinities of each ” is to be product. Thus when aldehyde is oxidized in presence of deprecated. The same may be said of the conventional nitric acid, perhaps a nitrate is first produced which loses method of formulating such compounds. Julius Thomsen water, and the compound thus formed then combines with has argued from his thermochemical observations that the the elements of the water molecule in the reverse order, two carbon atoms in ethylene are even less firmly united than are those in the corresponding paraffin ethane, and giving rise to a derivative of glycol, that the carbon atoms in acetylene have a negative CH3 CH2 -> CH2(OH) affinity, but, as the writer has pointed out (Phil. Mag. Feb. CH(0H)(0.N02) CH(0N02) CH2(0N02), 1887), the argument is based on a fallacy. In point of which on oxidation yields glycollic acid, CH2(0H).C02H. fact, although the ethenoid configuration is an unstable That oscillatory reversible changes are of far more one, there is no reason to suppose that the carbon atoms frequent occurrence than is commonly supposed cannot are less firmly united than they are in ethane, but quite the contrary; the only conclusion to be safely deduced Reversible be doubted, and there is every reason to suppose changes ^ that many apparently obscure or abnormal cases from chemical considerations is that the ethenoids are often un- are to be explained as consequences of such unsaturated, and in consequence capable of entering readily perceived, changes. One or two such instances may into interactions. The problem to be solved in connexion be referred to in illustration. It is well known that with them is the precise manner in which the carbon paraffin radicles are readily introduced into benzene valencies operate effectively in such compounds; as yet and its homologues by the interaction of the benzene we have no clue. A noteworthy property of the acetyland an alkylic chloride in presence of aluminium chloride enes is that of forming metallic derivatives—a property —one of the most remarkable of condensing agents. It only observed in this class of hydrocarbons. It is is a peculiar property of this agent, however, that very commonly supposed that the hydrogen is directly displacefrequently the entering radicle takes up the meta- rather able by the metal, but it is more probable that it is only than the ortho- or para-position, which is that assumed indirectly displaced, and that the formation of metallic when milder agents producing similar effects—such as derivatives from such hydrocarbons is an outcome of their ferric chloride, and in some cases sulphuric acid are highly unsaturated condition—not of any specific property used. Aluminium chloride, however, is not only a most of the hydrogen—which renders them capable of associatpowerful synthetic agent, but—as its discoverers, Friedel ing with metallic compounds. The formation of sodium and Crafts, have shown—equally powerful as an analytic acetylide, for example, may be represented in the following agent, destroying as rpadily as it builds up. Apparently manner:— reversal takes place much more readily in the case of paraC2H2 + NaOH = C2H2(OH)Na = C2HNa + HOH. or ortho- than of meta-compounds. To take a specific case, that of the butylation of toluene—the initial product is In like manner it may be said that the displacement of probably a mixture of para- and meta-butyltoluene in the hydrogen in water by sodium is due to the attraction which the former largely preponderates, but the action of the sodium by the oxygen, and merely a secondary being reversible in the manner expressed by the equation phenomenon. If the facts taught by the study of carbon compounds C0H5.CH3 + BuC1 ^ C6H4(Bu).CH3 + HC1, and the paracompound being chiefly attacked as the action proceeds, the generally are considered from the point of view of the meta-compound is formed in an increasing proportion so lessons they convey as to the valency of that comelement, and as to the manner in which carbon piexltyof that ultimately little or no para-butyltoluene remains. carbon Regarding the behaviour of saturated compounds towards combines with carbon, it is clear that the moIecu,e agents generally from the point of view discussed in this immense variety of compounds may be referred to four simple types — the paraffinoid, the ethenoid, section, it is obviously analogous to that of benzenoid compounds; in fact there is no need to draw any radical the acetylenoid and the benzenoid—the first two indistinction between the two classes. In both cases action cluding both open and closed chain forms. In other takes place initially in much the same way, but different words carbon atoms tend to combine—to exert their four secondary changes supervene, chiefly owing to the fact units of affinity—in only four different ways. The maxithat in saturated compounds, usually two, and sometimes mum stability is attained in the cycloids, especially in three hydrogen atoms are associated with the carbon benzenoids. When the tendency of carbon to form benatom which is the centre of attack, but never more than zenoids of a high degree of complexity, and the great stability of these is taken into account, it appears to be one in the case of the benzenes. The primary change in all interactions in which the almost a necessary consequence that the carbon molecule ethenoid compounds are concerned appears always to be should itself be highly complex. Apparently it should one involving the conversion of the ethenoid, by be possible to “ fuse ” cycloid upon cycloid, much Ethenoid coinbination with the agent, into a saturated as cell may be added to cell in a honeycomb, and the stability of the system should increase rather than founds. (paraffinoid) compound. The action probably diminish with increased complexity, provided the. contakes place in two stages—in the first, the two molecules associate as wholes'} in the second, the two ditions which obtain in a compound like anthraquinono