the association, nor availed themselves of it as a means of classification. On the contrary, although the groupings of the elements are as real and certain as the natural families of plants and animals, yet we find constantly, in our systematic treatises, that bodies manifesting the strongest analogies are widely separated from one another, while bodies belonging to very different groups are conventionally associated. The existence of certain important natural families has been successively pointed out by different chemists. I propose to make some modifications in the groupings hitherto proposed, to construct a few new groups, and to point out the principal analogies by which the members of each particular group, old or new, are associated; relying chiefly upon well-known recorded facts, but occasionally introducing fresh experiments and obser vations. The most prominent relations which obtain among the elements are the relation of parity or equality, and the relation of series or gradation; but an aliquot or multiple relation is also occasionally manifested*. The relation of parity is illustrated in the case of the twin-metals nickel and cobalt, and the relation of series in the triad chlorine, bromine, and iodine. The multiple relation is not shown by any such glaring instance. Some apparently multiple relations may indeed be only uncompleted relations of gradation, of which occurrence palladium and platinum afford a possible example; while other numerically multiple relations are scarcely supported by analogy of properties, as happens with platinum and gold. Frequently, however, we find that two proportions of one metal are isomorphous with one proportion of another, as is the case with copper and silver. In attempting a natural classification of the elements, we must have regard, though not an equal regard, to all the properties they manifest; or in other words, we must be guided by the totality of their characters. If we find that two or more elements form a large number of compounds of analogous composition, and if, moreover, these analogous compounds present a marked similarity of properties, we shall generally be justified in associating the elements in question, despite the generation by each of compounds to which the other furnishes no analogues. Moreover, if we find a marked general accordance in properties, we must overlook a discrepancy in some one particular set of properties, or regard it as pertaining only to the imperfect state of our knowledge. In considering the relation of gradation more especially, we must be careful not to mistake differences in degree for differences in kind. The protosalts of iron and copper show *These relations correspond generally with those pointed out by Gladstone, Phil. Mag. S. 4. May 1853. a general correspondence; but iron manifests a tendency to form persalts, copper to form subsalts, and hence the two metals are usually widely separated in our classifications. I hope to show that they form with nickel a most characteristic natural family. Again, arsenic produces acid oxides, and bismuth basic oxides; but the antagonism disappears on the introduction of antimony as an intermediate term. I am inclined, indeed, to place great confidence in gradational differences, not as a means for severance, but for association. That certain natural families have not long ago been recognized and made available in practice, has arisen, I believe, from the accordance of an undue consideration to certain unimportant differences. Our attention has been arrested by the most superficial rather than by the most fundamental characters. For instance, the sulphydrate of magnesium is soluble in water, that of zinc in acidulated water, that of cadmium in moderately strong acids. Hence the three metals, despite their great analogies, have been referred to three artificial classes. The point I wish to establish is, that, in our attempts at classification, we must rely upon natural characters, and not upon some arbitrary rule, such as the degree of temperature at which a metal decomposes water, or the strength of acid in which a metallic sulphide is soluble. Doubts may ever arise as to the proper distribution of some particular element, but the existence of natural families of elements, to which all artificial arrangements must give place, is I conceive indisputable. GROUP I. Fluorine-Chlorine, Bromine, Iodine. These four substances have one marked property in common, a property not pertaining to any other element with which we are acquainted, namely, that of combining with hydrogen in the proportion of atom to atom, gaseous atomic volume to gaseous atomic volume. The combinations moreover take place without any condensation; the resulting compounds are powerful acids, and exhibit a general resemblance in their characters and behaviour. Chlorine, bromine, and iodine present a marked similarity and gradation of properties. Fluorine is separated from the other members of the group by certain specialities. The atomic weights, which also express the vapour densities referred to hydrogen, form a remarkable sequence* : * The atomic numbers made use of in this paper are those employed by Phil. Mag. S. 4. Vol. 13. No. 88. June 1857. 2 G The atomic weight and specific gravity of fluorine, or 19, is rather more than half that of chlorine. Chlorine, bromine and iodine, when in the liquid or solid state, have, according to Kopp, the same atomic volume, namely 160. Bromine and iodine have sensibly the same atomic heat; 6.7 and 6.8 respectively. All four bodies in the free state manifest powerful affinities. Fluorine, however, appears to be by far the most energetic. Chlorine, bromine, and iodine are capable of replacing hydrogen by equivalent substitution in a great variety of compounds, mineral and organic; in the latter, more especially, with but a slight modification in the properties of the original bodies. Analogous fluorides, chlorides, bromides, and iodides are for the most part isomorphous. The fluorides, chlorides, &c. of hydrogen and the basylous metals have a great tendency to unite with the protochlorides, bichlorides, and terchlorides, &c. of the chlorous metals. The chloride, bromide, and iodide of hydrogen yield a large number of corresponding oxides, thus: : Chlorine moreover forms a peroxide, CIO2, or rather ClO2.CIO2 =C1204. Neither fluorine nor fluoride of hydrogen yields oxygenized compounds. Fluorine is moreover distinguished from the other three members of this group by its capability of reacting upon and dissolving certain insoluble mineral acids, such as the silicic, columbic, tungstic, &c., and by its greater tendency to form insoluble salts. Fluoride of silver, however, is very soluble. Iodine is possessed of metallic opacity, brilliancy and colour. GROUP II. Oxygen-Sulphur, Selenium, Tellurium. An atom of each of these elements unites with two atoms of hydrogen to form a compound which is either neutral or feebly Dr. Miller in his Elements of Chemistry,' save that in some cases they have undergone multiplication so as to correspond with M. Gerhardt's system of formulæ. * Laurent's 'Chemical Method,' Cavendish Society's translation, pp. 483 and 349. acid. One gaseous atomic volume of the element unites with two gaseous atomic volumes of hydrogen to form two gaseous volumes of the compound,-the condensation consequently amounting to one-third. Sulphur, selenium, and tellurium exhibit a marked similarity and gradation of properties. Oxygen manifests a general resemblance to the other three terms, and appears to bear towards them much the same relation that fluorine bears to chlorine, bromine and iodine. The atomic weights and vapour-densities occur in sequence :— The atomic weight and specific gravity of oxygen, or 16, is exactly half that of sulphur. The specific heat of the atoms of sulphur, selenium and tellurium, is double that of most of the metals*. The atomic volumes of sulphur and selenium, or 101 and 103 respectively, are sensibly equal. The number for tellurium is 128, the want of relation being possibly due to a want of correspondence in allotropic condition. Oxygen and sulphur, possibly also selenium and tellurium, unite with hydrogen in a second, and probably in several other proportions. The series, including some derived members, is as follows: C12 S Chloride of sulphur. their derivatives, unite with oxygen in several proportions: Regnault, Annales de Chimie et de Physique, 3 ser. vol. xlvi. p. 257. H2SO3 or H2O.SO2 H2 SeO3 or H2O.SeO2 H2TO3 or H2O.TO Sulphurous acid. H2SO4 or H2O. SO3 H2 SeO4 or H2O.SeO3 H2 TO4 or H2O.TO3 Sulphuric acid. In each of the above, and probably in all bihydrogenized compounds, one or both atoms of hydrogen may be replaced by a metal. The sulphides of hydrogen and alkaline metal are capable of uniting with six atoms of oxygen to form the following series of acids and salts : : The corresponding oxides or anhydrides of sulphur, selenium and tellurium, appear to resemble one another in properties: Corresponding compounds of sulphur, selenium and tellurium, are isomorphous. The isomorphism of oxygen with sulphur is not well marked. The majority of the metallic sulphides, however, crystallize in the regular system, and are so far isomorphous with the corresponding oxides. The number and variety of analogous compounds to which sulphur and oxygen give origin are very great: The bodies H2O2 and H2 S2 are binary molecules, HO.HO and HS. HS respectively. HO is not, however, in any way CNKS K2 CS3 N2 S2 K3 POS PC13S SbS3 |