elements are equal, if the molecule is in each case referred to 2 volumes of vapour.* If, now, we consider it proved, from the above considerations, that the molecular weights of carbon, silicon, tin, titanium, and zirconium, correspond in the gaseous form to two volumes, and that these elements are tetratomic, it follows from analogy that certain other elements are also tetratomic, and we have the group, The first column of numbers giving the atomic weights; the second the atomic heats, or the products of the atomic weights into the specific heats. It is possible that thorium belongs to the tetratomic group, as thoria appears to be isomorphous with stannic acid, Sn, O. Lead is placed in this group from the analogy of the compounds which it forms with ethyl, methyl, &c., to stan-methyl, &c. It appears, however, like tin, palladium, ruthenium, platinum, rhodium, iridium, and osmium, to be also diatomic; the so-called protoxides having the formulæ Pb, O,, Sn, O2, &c. For similar reasons we admit the existence of six triatomic elements, namely: The diatomic elements, according to the reasoning above mentioned, will be thirty in number, and may be arranged in natural families, as follows:: *The only exception to this law occurs in the case of carbon, and cannot at present be explained. This classification into natural families appears to me to represent the present state of our knowledge, though the position of several elements must be regarded as doubtful.* The monatomic elements are only ten in number. They may be arranged in three groups : Chlorine, 35.5 Hydrogen, 1 Silver, 108 6.16 It is, however, to be remarked, that, of the other elements, at least two-namely, copper and mercury—are monatomic in certain combinations, as, for example, in Cu, Cl and Hg, Cl. It is true that we may write these formulæ Cu, Cl2, and Hg, Cl,, in which case we have two additional diatomic forms of copper and mercury, with the atomic weights 126 and 400 respectively. This mode of viewing the subject obliges us to admit atomic heats, represented nearly by the number 12, or twice as high as in the case of the other elements, and appears, therefore, less simple than that first stated. In like manner, all the elements belonging to the tetratomic group which form protoxides, &c., may be regarded as tetratomic and diatomic; this will add at least eight to the number of diatomic elements. Finally, in the sesquioxides, and similar compounds, two received atoms of * There is reason to believe that vanadium belongs, with boron, to the nitrogen group. iron, manganese, chromium, aluminum, &c., must be regarded as forming a single triatomic atom. Limpricht and Frankland admit the existence of pentatomic elements, including the nitrogen group in this class; but this view is not generally adopted, and in the present state of our knowledge, a primary division of the 60 elements into four classes, the monatomic, diatomic, triatomic, and tetratomic elements, appears to be sufficient. From the above it appears that of the sixty elements now known to chemists, the received atomic weights of fifty at least must be doubled, if we admit the correctness of the reasoning which has led many chemists at the present day to double the atomic weights of carbon, oxygen, and sulphur. Wanklyn and Wurtz have already suggested the propriety of doubling the equivalents of zinc and tin, and, by analogy, of several other metals, while Cannizzaro has adduced the specific heats of the atoms, with other arguments, in favour of the same change. Wurtz compares oxide of zinc, Zn, O2, with oxide of ethylene (C1 H1) O.; the hydrate of oxide of zinc will then Zn, correspond to glycol, HO.. In the same way we may H2 consider hydrate of sesquioxide of iron as corresponding to glycerine, Fr, 0, to H1 CH, HO., Fe,being heretriatomic. Should the further progress of the science show that the views above mentioned are the most simple and consistent expression of all these facts, it will be desirable to return to the notation of Berzelius slightly modified, that is to say, it will be better to halve the atomic weights of hydrogen, and the other monatomic and triatomic elements, and denote the atomic weights of carbon, oxygen, sulphur, zinc, &c., by the numbers 6, 8, 16, 32, 5, &c., respectively. In this manner, while the formulæ are written with precisely the notation of the new school, the numbers actually employed in the great majority of calculations will be those which have been sanctioned by longest usage, and which are most convenient for practical purposes. The atomic weights of hydrogen, chlorine, potassium, and the other monatomic elements, will be in each case one-half of the old equivalent, while the molecular weights will correspond with the old equivalents. Thus we shall have, In which table the first column gives the atomic, and the second the molecular weights. The formula of water thus becomes H, O, and its atomic K) weight 9; caustic potash is O, and its atomic weight 28; oxide of zinc will be ZnO, and its atomic weight 405; chlorhydric acid will be H Cl, and its atomic weight 18.25. The dashed symbols H, €, K, &c., &c., may also be employed as in the Berzelian notation, and would in many cases be extremely convenient. All the typical formulæ now so generally employed will be written as at present, the actual weights only being changed. The general acceptance of the views of the new school would be greatly facilitated by the adoption of the system of atomic weights here proposed.* CHAPTER VIII. WATER TYPE. 541. The substances treated under the simple and complex molecules are subdivided into two groups,-the positive and negative. 542. This group embraces-(1) the metallic derivatives of water; (2) the basic derivatives of ammonia, constructed on the water type; (3) the alcohols and ethers. (1.) HYDRATES OF THE METALS PROPER (PRIMARY DerivaTIVES). ANHYDROUS OXIDES (Secondary Derivatives). * American Journal of Science and Art, vol. xxxi. No. 92. 543. Primary derivatives. When one-half the hydrogen in water is replaced by a positive metal, the hydrated base or the primary derivative is produced. The semiatomic metals replace the hydrogen of the water in the proportion of two equivalents of the metal for every one of hydrogen; the hydrates of these semiatomic metals are formed on the type of a single molecule of water; their general formula is therefore M, "H } 0. 544. All those metals which we have given, under the simple molecule of hydrogen, as monatomic, form hydrates on the type of a single molecule of water; their general formula is therefore 0. M H 545. Secondary derivatives.—When all the hydrogen in water is replaced by a metal, then we get the anhydrous oxides of the metals, or the secondary derivatives. The general formula of the anhydrous oxides of the semiM2) atomic metals is "M, O, and the general formula of the M2 M anhydrous oxides of the monatomic metals is MO, as the dinoxides and the protoxides are each formed on the type of a single molecule of water. H 546. The sulphides, the selenides, and tellurides, are constructed on the water type, in which the oxygen is replaced by sulphur, selenium, and tellurium; the formula of sulphide of hydrogen is therefore S, the general formula of the primary derivatives, or hydro-sulphurets, on the simple molecule, is S, and the sulphides, or secon M H dary derivatives, is MS. } H EXERCISE. 134. Write out the formula for hydrate of strontium, for the protoxide of barium, for the hydrosulphuret of potassium, for the dinoxide of mercury, for the selenide of sodium, for the telluride of calcium, for the sulphide of sodium. |