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733 CHEMISTRY are satisfied, i.e., that the cycloids be so arranged as to planes at right angles to the ring plane. Numerous ingive rise only to centric and saturated (paraffinoid) hexa- stances of this kind of isomerism are presented by the cycloids, and to exclude the formation of ethenoid rings such benzene-carboxylic acids, which have been studied more as presumably occur in anthracene. It is possible even particularly by von Baeyer. to go farther, and from this point of view to explain the Enough will have been said to make clear the use that differences observed between the different forms of carbon. is made of the conception that the affinities of carbon are The diamond may well be a complex of the anthraquinone directed effects. The hypothesis has been of the utmost type, while both graphite and amorphous carbon may service, as with its aid it is possible to give a rational more nearly resemble anthracene in structure, the low explanation of every case of isomerism hitherto described. stability of the former as compared with the latter being But after all the model used is but a model, and probably due to the presence in amorphous carbon of a greater a very imperfect expression of, if indeed it in any way number of ethenoid rings. Such assumptions would prob- approaches to, the actual state. There is little doubt that ably also serve to account for the colourlessness of the what is true of carbon is true of many if not of all the diamond, and for the opacity of the allotropic forms. other polyad elements. For like reasons it is probable that amorphous carbon will be found to be the most, and diamond the least, refractive Chemical Change. of the three forms. In order that the marginal affinities in a system of cycloids extending laterally may be satisChemical text-books teem with equations representing fied, it is obviously necessary to assume that the cycloids changes as occurring between pairs of substances, so that may be arranged in tWo or more planes, i.e., in layers. It from the outset of his career the student becomes imbued is conceivable that in a system in which the marginal with the notion that such expressions are faithful repreelements were all either centric or ethenoid, the marginal sentations of the facts—especially as he witnesses many affinities to be satisfied would all lie in a single plane; experiments in which apparently two substances are but if the marginal elements were partly paraffinoid they caused to interact; he is rarely made aware that the would not all lie in a single plane. It is well known materials used are always more or less impure—that a that the diamond as well as amorphous carbon at high pure substance in fact is, and must ever be, .an ideal temperatures is converted into graphite, which is the conception, and he is still less rarely led to reflect on the most stable of the three forms: it may well be that possible influence of “ impurity ” as causative of change. the change which takes place is that of a system of Yet, of late years, instance after instance has been brought centric and paraffinoid elements into one in which the forward of a “ third substance ” playing an all-important marginal paraffinoid elements become either centric or and indeed determinative part in conditioning the chemical ethenoid. interchange; so much is this the case that it is now seen The conception that the carbon atom may be sym- to be more than probable that every interchange involves bolized by a tetrahedron affords an explanation of numerous the co-operation of a “ third ” substance. cases of isomerism besides those in which an Many years ago Wanklyn stated (Chem. News, xx. 271) trans-0*1 asymmetric arrangement of the radicles such as that dried chlorine is without action on sodium even when isomerism, would condition optical activity is to be pos- the latter is heated; but until Cowper pointed out, presence tulated. In the case of ethenoid derivatives, in in 1883 (Trans. Chem. Soc. 1883, p. 153), that of third which a pair of carbon atoms are associated by two affini- while dried chlorine is without action on copper substance ties of each, the affinities uniting the two atoms lie in one and a number of other metals, the moist gas at necessaryplane, whilst the remaining four affinities lie in a second once attacks them, the influence of moisture in promoting j, plane at right angles to the first. Hence an such interactions did not attract attention. In 1884 the j,1 || p2 ethenoid compound may be represented by important discovery was made by H. B. Dixon (Phil. 3 4 such a symbol as is shown in the margin, in Trans. 1884, p. 617), that a dried mixture of carbonic which the double vertical line indicates the “double oxide and oxygen gases does not explode, but that a mere bond ” between the carbon atoms. trace of steam renders it explosive, the rate at which the In such a case isomerism will arise if merely the position explosive wave travels in such a mixture being dependent, of R3R4 relatively to R4R2 be varied, the radicles lying in up to a certain point, on the amount of moisture present. the one case on the same, and in the other on opposite H. B. Baker then showed (Phil. Trans. 1888, p. 571) sides of the plane uniting the two carbon atoms. Iso- that when moisture is, as far as possible, excluded, carbon, merides of this kind are not optically-active, as the radicles sulphur and phosphorus can be heated in an atmosphere are all situated in one plane. So simple a difference in of oxygen without undergoing visible combustion. Holt structure often involves a considerable difference in pro- and Sims subsequently observed that dried oxygen does perties, as in the case of maleic and fumaric acids, for not oxidize either sodium or potassium (Trans. Chem. Soc. example. Of these the former alone yields an anhydride, 1894, p. 432). In 1893 H. B. Baker extended his exand is therefore represented as the cis-form by symbol 1, periments to compounds, and made the very remarkable fumaric acid, the trans-form being represented by symbol 2 discovery that neither does oxygen combine with nitric oxide nor ammonia with hydrogen chloride when the gases CH.COOH C09H.CH are very carefully dried. The same observer has found (1) II (2) ' II that the reverse effect—the occurrence of decomposition— CH.COOH; CH.COOH. is also affected by the presence of moisture, it being Cis- and trans-isomerism occurs, it is supposed, not only possible, for example, to vaporize ammonium chloride and among ethenoid compounds, but also in the case of mercurous chloride without their undergoing decomposisaturated ring systems generally; in fact, the ethenoid tion if moisture be rigorously excluded. The utmost care, compounds present but a special case of this kind of however, must be taken to secure such a result, it being isomerism. In a ring system — whether composed of necessary, for instance, in determining the density (by carbon atoms alone or of such in conjunction with other Y. Meyer’s method) to use an apparatus constructed of polyad elements—the affinities of the atoms which are hard glass, since soft glass apparently retains sufficient united within the ring lie in the plane of the ring, and moisture to invalidate the experiment. In some cases, the two remaining affinities of each carbon atom lie in [ although it has not been found possible to prevent the