should be doubled; the equivalents of these bodies would then become Zn=66; Hg= 200; Sn: 118.* We have already noticed (198 and 208) the way these compounds of metals and organic radicals are prepared.† 533. The hydrogen compounds of these biatomic nonmetallic elements are constructed on the type of water, and they are, therefore, noticed under that type. Aldehydes of the bibasic mineral acids. Hydrosulphuric acid, or aldehyde of hyposulphurous acid Hydroselenic acid, or aldehyde of hyposelenious acid H, S H, Se Aldehyde of sulphurous acid (not yet discovered) H2SO Aldehyde of sulphuric acid (not yet discovered) H2SO, TREBLE Molecule. H1, H,. POSITIVE GROUP. HYDRIDES OF METALS (PRIMARY DERIVATIVES). 534. Primary derivatives.-The hydrides of arsenic and antimony are constructed on the type ammonia, and therefore they are considered under that type. 535. Secondary derivatives.-Several metals can replace hydrogen by interchange of one atom of metal for three atoms of hydrogen; these metals are principally arsenic, antimony, bismuth, molybdenum, vanadium, tungsten, See Appendix B, page 279. + Than and Wanklyn have attempted, but hitherto without success, to form combinations of the metals with the biatomic radicals of the olefiant gas series. When they acted upon iodide of ethylene with zinc and other metals, an iodide of the metal was formed, and olefiant gas evolved. student is referred, for further information, to their paper in the Quarterly Journal of the Chemical Society, vol. xii. page 258. The and gold; gold is the only metal of this group which is known to act sometimes as a monatomic metal. Arsenic, antimony, and bismuth will be noticed again under the negative group, and the others, with the exception of gold, on account of their rarity may be disregarded. The symbol for the molecule of gold constructed on the treble type will be Au"", Au””. 536. Many metals can replace hydrogen by interchange. of two atoms of metal for three atoms of hydrogen; "the principal metals which have this property are aluminum, cerium, uranium, chromium, iron, manganese, nickel, and cobalt, in which last two, however, the tendency is but slight. Aluminum alone is not known to replace hydrogen in any other than the above proportion. The other metals form proto, as well as sesqui-compounds. It is sometimes convenient to replace the double atom of metal in sesqui-compounds, by doubling one of the letters of its symbol, thus: All, Ffe, Mmn, Uu, Cer, &c., instead of Al,, Fe, Mn,, U2, Ce,, &c. The property which these double or twin atoms have of replacing three atoms of hydrogen, may then be conveniently indicated by the use of three dashes placed to the right of the symbols, thus:-Ffe", All", &c." The symbol of the molecule would then be thus expressed Ffe"", Ffe"; All", All""; Ccr", Cer". COMPOUNDS OF TRIATOMIC METALS WITH MONATOMIC RADICALS. 537. Aluminum treated with iodide of ethyl or iodide of methyl yields, at temperatures from 100 to 130° C., double compounds, aluminum-ethyl and iodide of ethyl, or aluminum-methyl and iodide of methyl; the organoaluminum bodies have not yet been obtained with certainty free from iodide of aluminum. The following are the reactions: C2H 3 (C2 H ̧, I) + Al,= (All" C, H, + All”, I ̧) C2H, (CH, 3 (C H,, I) + Al, = (All" CH3 + All”' I3). NEGATIVE Group. HYDRIDES OF NEGATIVE OR ACID METALS (PRIMARY DERIVATIVES). SECONDARY DERIVATIVES. 538. Primary derivatives.-Arsenic, antimony, phosphorus, and nitrogen form terhydrides, which distinguishes them from all other elements; bismuth, which is, like arsenic, &c., a triatomic element, does not appear to form a hydride, but it forms, like the other triatomic elements, a terchloride. The following are the hydrides of this group, ammonia, which is one of the hydrides, is a typical body, and the other three hydrides are considered to be formed on the ammonia type; they will, therefore, all be considered under the type ammonia. The following are the symbols for these molecules : H1, N"; H3, P"; H3, As"; H, Sb”. 539. The combinations of the organic radicals with nitrogen, phosphorus, arsenic, and antimony will also be considered under the ammonia type. QUADRUPLE MOLECULE. H1, H. 540. The members of the silicon group (pp. 165 and 168) are tetratomic bodies; two of them, tin and titanium, sometimes act, as we have noticed, as biatomic elements; carbon and silicon form hydrides; marsh gas, which we have previously placed amongst the hydrides of the alcohol radicals as the hydride of methyl, may also be formulated on the quadruple molecule thus, H, C. The composition of silicide of hydrogen has not been ascertained; but it is obtained by the action of hydrochloric acid on silicide of magnesium, and as the composition of this silicide is Mg. Si, we may infer that the hydrogen compound is H, Si. APPENDIX A. Homologous bodies.-The term homologous, which was first employed by Gerhardt, is applied to a series of analogous substances which fulfil the same chemical functions, follow the same laws of metamorphosis, and contain in their molecule n times CH,, more or less. The decomposition products of homologous bodies, when these bodies are decomposed by the same agents and under the same circumstances, are also homologous. Isologous bodies.-Bodies are termed isologous when they belong to different groups of the same class of bodies. Bodies, although allied in properties, are placed in different groups of the same class, if they differ in chemical composition. For example, the student will find that there are different groups of hydrides of alcohol radicals— different groups of alcohols; the members of the same group are homologous-the members of different groups of the same class of bodies are isologous. Example : Methylic alcohol is an homologue of vinic alcohol, for they differ n times CH, in their composition, and one general formula will express the composition of both bodies, thus:: But benzoic alcohol is not an homologue, but an isologue. It belongs to a different, not to the same, group of alcohols to which vinic and methylic alcohol belong, for it does not differ n times C H, in its composition, and one general formula could not express the composition of these different alcohols. We shall here give a further quotation from Dr. Wolcott Gibbs's excellent paper on the atomic weights of the ele ments: If we admit that the molecules of all substances occupy two volumes in the gaseous state, and if the reasoning which has led chemists to double the old atomic weights of carbon, oxygen, sulphur, &c., be correct, it follows that the atomic weights of the greater number of the elements must be doubled. For we find,— 1. That nearly all volatile inorganic oxides, chlorides, oxychlorides, &c., contain, in two volumes of vapour (H1), two or four received equivalents of metal or radical. Thus the compounds whose formulæ are now usually written Si Cl2, Ti Cl2, Z Cl2, Sn Cl,, Hg Cl, Hg, Cl, Hg Br, Hg, Br, Fe, Cl, Al, Cl,, Al, I ̧, Al, Br, Cr O2 Cl, must be written Si, Cl, Ti, Cl, Z, CI, Sn, Ci, Hg, Cl2, Hg, Cl2, Hg, Br2, Hg, I2, Fe, Cle, Al, Cle, Al, I., Al, Bre, Cr2 O, Cl2, in order to correspond to two volumes of vapour. 2. All volatile metals and metallic oxides contain two or four received equivalents in two volumes of vapour. Thus the vapour-densities of mercury and cadmium represent the molecular weights, Hg2, Cd. Arsenious and osmic acids correspond, in the form of vapour, to the formulæ As, O, and Os, О ̧. 3. All volatile compounds of metals with organic radicals contain an even number of equivalents of metal in two volumes of vapour, excepting only those in which triatomic metals occur. Thus we have, corresponding to two volumes of vapour, the formulæ,— C, H, H, Zn, C. Zn, (C,H,). Sn, (C ̧H,), Sn2 4. The volatile compounds of the triatomic radicals, nitrogen, arsenic, antimony, bismuth, and boron, contain in two volumes of vapour only one received atomic weight of the radical. Thus we have the compounds : NH, PC, As Cl,, Sb Cl, Bi Cl,, B Cl,, P (C, H2)s, As (C, H,),, Sb (C, H ̧), B (C, Hs), BO,, 3 C, H, O, &c., &c. To this there are exceptions. Thus we have, corresponding to two volumes, the formula, 5. The specific heats of the atoms of nearly all the |