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CHEMISTRY 715 remarkable in view of the impression which a study of tures far higher than those which platinum will withstand. nitrogen compounds conveys that the element is a highly In using the apparatus, the bulb is heated to a conactive one. An extreme instance is afforded by carbon. stant temperature, which is easily ascertained by watching No conception of the properties of this element can be whether air—or whatever gas the apparatus may contain obtained by studying it in any of the forms in which it is escapes from the side tube ] the stopper is then withordinarily met with, and the reason is not far to seek. As drawn, a weighed quantity of the substance to be examined ordinary amorphous carbon is converted by oxidation into is deftly introduced into the tube, and the stopper reinmellitic acid, C6(C02H)ti, a derivative of benzene, it may serted. When the substance reaches the bottom of the be assumed that it contains at least 12 atoms in its bulb it quickly volatilizes, and displaces the gas from the molecule; but mellitic acid contains a ring composed of bulb;.this is collected and measured. There is no need 6 atoms of carbon—the benzene ring, and for the reason, to notice the temperature. The gas collected is measured among others, that there is an obvious tendency on the under the prevailing atmospheric conditions; its volume part of carbon to form such rings, it is not improb- is that of. the vapour derived from the substance, asable that each of the six atoms associated with the ring in suming this to have been gasified under atmospheric mellitic acid was originally present in the carbon in a conditions. benzene ring which underwent destruction during oxidation. By means of this apparatus, Victor Meyer, in conjunction On this assumption the carbon molecule contains at least with Carl Meyer, was led in 1879 to make the altogether 42 atoms of carbon. What is true of elements is equally unexpected discovery that chlorine has a much lower density true of compounds—especially of oxides. Thus, whereas at high than at low temperatures. The experiment was carbon and silicon yield comparable chlorides of simple made by dropping a known weight of platinous chloride composition, CC14 and SiCl4, their oxides are totally into the bulb; this underwent decomposition into solid different, carbon dioxide being gaseous, whilst silicon platinum and gaseous chlorine. At temperatures between dioxide is a non-volatile solid that fuses with difficulty. 12.00 and 1500 the value obtained was only about twoThe difference is undoubtedly due to the fact that the thirds of that to be expected on the assumption that molecule of.the latter is complex, x in the formula (SiO.,)* ordinary chlorine was dealt with. Crafts and Meier, who having possibly a high value. Another striking illustration repeated the Meyers’ experiments, using ordinary „ ’ , of the dependence of properties on molecular composition chlorine, did not confirm their results. But dllsociaf is afforded by the two oxides of nitrogen, NO., and N.,0 ; both parties were right. It is now generally Mon by the former is an intensely coloured gas characterized by an admitted that when the halogens are strongly absorption spectrum of extraordinary complexity, whereas heated the ordinary diatomic molecules undergo dissociathe latter is a colourless crystalline solid. Although the tion into monatomic molecules, e.g., force of such facts has long been recognized, they are by no means sufficiently brought under the notice of students. J2 ^ J + J. hi en in the case of gases, although Avogadro’s theorem This change takes place comparatively readily in the case is accepted as the basis of our system, its consequences are of iodine, much less easily in the case of bromine, and only too often disregarded in the text-books, many of which are to a very slight extent when ordinary gaseous chlorine is disfigured by the appearance of H, O, and N as symbols of heated. When platinous chloride is strongly heated, howhydrogen, oxygen, and nitrogen, for example. Canizzaro’s ever, the chlorine is initially separated from the compound striking Faraday lecture, delivered in 1872 (Journ. Chem. in the monatomic state, and hence it is that much lower Soc. pp. 941-967), dealing specially with the application of values are then obtained. The same is true of platinous Avogadro’s theorem, may be consulted with advantage by bromide as compared with ordinary bromine. The result all who wish to gain correct views on such a question. is one of great interest theoretically, and justifies the Ultimately all peculiarities such as have been referred assumption often made that in the nascent state substances to must be traceable to peculiarities inherent in the ele- may exercise different and more powerful effects than ments , in order that this may be done, and that the extent they do ordinarily. Meyer found that the complex (tetrato which properties depend on molecular composition tomic) molecules of phosphorus and arsenic also undergo may be ascertained, the determination of molecular weight simplification at a white heat—the values approaching becomes an operation of the first importance, and it is not those calculated for diatomic molecules. When zinc and fo be forgotten that the determination of molecular weight bismuth are volatilized, they—like mercury and cadmium is a necessary step in the determination of atomic weight. ——afford monatomic molecules; thallium, however, yields Very great improvements in the methods of determining diatomic molecules. Numerous attempts have been made .gaseous densities have been effected in recent times, to determine the vapour densities of the alkali metals, but Meyer’s by Victor Meyer, and whereas formerly unfortunately they attack both platinum and porcelain method. ^ was on^y possible to deal with gases, methods vessels, and it is difficult to place confidence in the results. are now available for the determination of the The “ freezing - point ” and “boiling-point” methods molecular weights of liquids and of solids in solution. referred to later have been applied to a number of The form of apparatus introduced by Victor Meyer (in the (heavy) metals in the form of alloys or amalgams. 18//) is known and in use everywhere. It consists of The results seem to show that, in most cases, the liquid a cylindrical bulb attached to a long narrow stem, which metal consists of monatomic molecules. Speaking generis closed at the upper end by a stopper. Near this ally, in fact, there seems to be a far greater tendency on end a side tube is sealed on through which gas can escape the part of non-metals than of metals to form complex and be collected. By surrounding the bulb and part of molecules. It is more than probable that many of the the stem. with the vapour from a liquid of constant properties connected with, and indeed characteristic of, the boiling-point, or by placing it in a furnace, the temperature true metallic state have their origin in this circumstance. may be raised to any desired point. The only limit is that It is also not unlikely that important variations in the imposed by the material of which the apparatus is conproperties of metals, such as are occasioned by the presence structed , if porcelain or platinum be used, observations can of small amounts of foreign substances, may be the outbe conducted at very high temperatures. At the time of his come of changes in molecular structure. Osmond has death Meyer was engaged in attempts to construct vessels made this assumption in the case of iron, for example, to which would permit of experiments being made at tempera- explain the eUstence of hard and soft varieties, and it is