readiness with which definite chemical changes may be started among the constituents of a molecular group appears to shew that although these constituents are held together but loosely, nevertheless they are not merely mixed. Thus, As(CH),Cl combines with Cl, to form As(CH,),Cl,; when this compound is heated it yields As(CH,)C1, + CH,Cl; then As(CH ̧)Cl, readily takes up Cl, to form As(CH,)Cl, which on being heated separates into AsCl, + CH,Cl. Now on account of their properties some of these compounds must be classed as molecular, yet under the influence of heat the parts of the molecular groups mutually act and react in a way analogous to, if not identical with, that characteristic of chemical change. But such phenomena as these are exactly what might be expected from the hypothesis of molecular compounds; if these bodies are formed of groups of molecules we should expect that reactions between these groups would, in many cases, easily occur and result in the production of new, less complex, groups, or, it may be, new molecules. That a substance is found to behave in a definite manner under the influence of this or that reagent cannot be regarded as sufficient evidence for classing it among atomic rather than molecular compounds. Thus the observation recorded by R. W. Atkinson' regarding the identity of the salts produced by mixing SbCl, and 3KBr, and SbBr, and 3KCl, cannot be regarded as proving that the product of these actions is built up of molecules represented by the formula #SbCl,Br,K,, the properties of which are conditioned only by the mutual interactions of the atoms Sb, Cl, Br, and K. Regarded however as a contribution towards solving the questions suggested by the term molecular compounds, the observations made by Atkinson are of interest, as shewing how possible it is to obtain substances which behave in some respects as molecular and in other respects as atomic compounds. It cannot be too strongly insisted on that no hypothesis has been proposed regarding molecular compounds which furnishes us with a definition of the class 'molecular', or puts into our hands 1 C. S. Journal, Trans. for 1883. 289. an instrument for determining whether a given compound belongs to this class or to the class of atomic compounds. What the hypothesis does is to negative the notion that the properties of all compounds are to be explained by the conception of actions and reactions between atoms which together constitute a molecule, to restrict the application of the theory of valency to gaseous compounds, and to open a path for future research by insisting on the complexity of chemical phenomena, and the folly of attempting to explain all in terms of a favourite theory. But the consideration of molecular compounds leads to the discussion of questions which properly belong to chemical kinetics; we cannot separate these bodies from their environment; they are members of a system which is continually undergoing change and the comparative stability of which is the result of never ceasing action and reaction between its parts. Chemistry is not a collection of facts regarding the crystalline form, melting points, boiling points, specific gravities, &c., &c., of so-called pure elements or compounds; it is rather the orderly and regulated study of the changes which matter undergoes and which result in more or less profound modifications in the properties of the changing bodies. A great advance has certainly been made by replacing the conception of a molecule as an undefined quantity of matter constructed of groups of atoms more or less loosely and vaguely arranged, by that conception of the molecule which regards it as a definite and definable quantity of matter, built up of atoms arranged in an orderly manner, and exhibiting functions dependent on the nature, arrangement, and mutual interactions of these atoms. Among the functions of these molecules we must however, I think, place the power of combining with other molecules to form more or less complex groups, less stable than the molecules of a gas, and not so sharply defined from other groups as the molecule of one compound is from that of another. Although the explanation of the properties of molecular compounds is not to be brought within the scope of the theory of valency, nevertheless if we regard the formation (or nonformation), and the relative stabilities, of such compounds as functions of all the molecules concerned in their synthesis, we can see that the valencies of the elementary atoms must be important factors in determining the production of molecular compounds'. 1 In connection with this subject compare van't Hoff's Ansichten über die Organische Chemie, PP. 4, 5. CHAPTER III. THE PERIODIC LAW. 104. ATTEMPTS have from time to time been made throughout the preceding 50 or 60 years to trace connections between the atomic weights and the general properties of groups of elements. Soon after the appearance of Dalton's New System of Chemical Philosophy, an hypothesis was promulgated by Prout to the effect that the atomic weights of the elements are whole multiples of that of hydrogen; but the researches of Berzelius, Marignac and Stas shewed that this hypothesis was untenable. A modification of Prout's hypothesis was made by Dumas which appears to have a fair probability in its favour. Gmelin, Dumas, Gladstone, Cooke, Kremers, Pettenkofer, Odling, and especially Newlands' (who was among the earliest workers in this field,) have drawn attention to points of connection between the properties and the atomic weights of elements. It is however especially to Mendelejeff' that we owe the systematic correlation of the atomic weights with the chemical and physical properties of the elements. Lothar Meyer' has also made important contributions to the same subject, and in his Modernen Theorien he has 1 Chem. News, 7. 70, and 10. 59, 94. 12. 83, 94. 13. 113. &c. Newlands' contributions to this subject have been gathered together and published in a small volume entitled 'On the Discovery of the Periodic Law' [Spon. 1884]. 2 Annalen, Suppl. Bd. 8. 133. See also Chem. News, Vols. 40 and 41. 3 Annalen, Suppl. Bd. 5. 129, and 7. 354 &c. gathered together the more important facts which have been established concerning the relation in question. 105. We may confidently say that a large probability has been established in favour of the hypothesis that the properties of the elements, and of the compounds of each element, are periodic functions of the atomic weights of the elements. Lothar Meyer puts the general statement of the "Periodic "Law" in this form'; 'if the elements are arranged in order of 'increasing atomic weights, the properties of these elements 'vary from member to member of the series, but return more 'or less nearly to the same value at certain fixed points in 'the series'. Let the elements be arranged in the order of their atomic weights; let this list of elements be (broadly) divided into series of sevens; let the members of the second series be placed under those of the first, those of the third under those of the second, and so on; and let the elements contained in a vertical column be called a group, those in a horizontal column being called a series. Then taking an element R, and calling the elements next before and after it in the same series X and Y respectively, and those in corresponding positions in the same group R' and R", we may say that atomic weight, density, and atomic volume of R= atomic wt., density, atomic vol. of X+the values of same constants for Y 2 and also that atomic weight, &c. of R= atomic weight, &c. of R'+ constants of R" 2 In the arrangement of the elements just described, each group corresponds, for the most part, with a natural family. This is more clearly shewn, and the relations between the atomic weights and the properties of the elements are more distinctly developed if certain gaps are supposed to exist in the list of elements. The following table' exhibits this arrangement of the elements. 1 Die Modernen Theorien, 4th Ed. p. 136. 2 Taken from a paper by B. Brauner in C. S. Journal Trans. for 1882. 78: atomic weights are stated in round numbers. |