HOME UNIVERSITY LIBRARY HOME UNIVERSITY LIBRARY OF MODERN KNOWLEDGE No. 90 Editors : HERBERT FISHER, M.A., F.B.A. PROF. GILBERT MURRAY, LiTT.D., LL.D., F.B.A. PROF. J. ARTHUR THOMSON, M.A. PROF. WILLIAM T. BREWSTER, M.A. THE HOME UNIVERSITY LIBEARY OF MODERN KNOWLEDGE i6mo cloth, 50 cents net, by mail 56 cents SCIENCE Already Published ANTHROPOLOGY By R. R. MARETT AN INTRODUCTION TO SCIENCE By J. ARTHUR THOMSON EVOLUTION By J. ARTHUR THOMSOI-J and PATRICK GEDDES SEX By PATRICK GEDDHS and J. ARTHUR THOMSON THE ANIMAL WORLD By atoms, but the grouping of the atoms within the molecule that is the deter- mining cause of individuality. A phenomenon which at first appeared at variance with all common sense notions the fact which stag- gered the early investigators in this field, viz., that two or more molecules might have precisely the same ultimate composition, and yet be quite distinct forms of matter, has become a commonplace doctrine in modern science in the light of the theory of chemical structure. Moreover, the development of structural chemistry has of late years led to the recognition of the internal mobility of atoms within molecules of structural con- figurations so delicately balanced that the atoms constituting the molecule may assume one or another of two quite different configura- tions according to the conditions to which it is exposed. To this phenomenon, the general term tautomerism is applied. Stereochemistry. Yet another step in the theory of chemical structure, and we are STEREO-CHEMISTRY 223 face to face with one of the most brilliant of modern achievements in the direction of bringing Chemistry into the category of the deductive sciences. The atoms composing a molecule must obviously form a group in space the configuration is not that of a congeries of points all lying in one plane, but a system occupying tridimensional space. This conception was first definitely applied to the structural formulation of carbon com- pounds by Le Bel and van't Hoff in 1874. The four " bonds " of the carbon atom, for example, may be represented by lines radiating symmetrically from the carbon atom as a centre. This is expressed geometrically by supposing that the carbon atom is in the centre of a regular tetrahedron, the points of the angles of which represent the terminations of the bonds to which are attached the other atoms or groups of atoms which build up the molecule. If the carbon atom, regarded from this point of view, is combined with four different atoms or groups of atoms, there then arises as a geometrical necessity the existence of two different space groupings of the same molecule which are non-superposable, and which are related to each other in the same way that an object is related to its reflected image in a mirror a right and a left-handed 224 CHEMISTRY isomerism quite incapable of being represented by any formula which ignores the space con- figuration of the atoms. This conception in its modern developments may almost be said to complete the theory of isomerism ; large numbers of cases in which the differences between compounds of the same ultimate composition cannot be expressed by plane surface structural formulae are now known to be cases of stereochemical isomerism. This newer development of the atomic theory known as Stereochemistry is gradually per- vading and making its influence felt through- out the whole domain of our science. The fundamental idea of space grouping is not easy to follow at first without the aid of models, but the modern student is being taught to handle these formulae which, by virtue of their rationality, are bound to dominate more and more all our notions concerning the structure of molecules. The possibilities of isomerism, regarded from the stereochemical point of view, natur- ally become more complex with the increase in the number of carbon atoms which comply with the conditions of asymmetry just defined. Here, again, is there close parallelism between deduction from the theory and observed facts an everlasting testimony to the fertility STEREO-CHEMISTRY 225 of the idea. Thus, the acid of sour milk, lactic acid, contains an asymmetric carbon atom, and exists in two stereochemical forms, as required by theory ; tartaric acid contained in the juice of grape and other fruits contains two asymmetric carbon atoms, and exists in three stereo-isomeric forms as required by theory. The group of sugars typified by grape sugar, or glucose, comprised under the formula C 6 H 12 O 6 , contain four asymmetric carbon atoms, and can exist in sixteen stereo-isomeric forms. Many of these sugars are natural pro- ducts ; and nearly the whole series of sixteen required by theory has been synthesized by Emil Fischer and his colleagues a veritable triumph of modern carbon chemistry. One of the chief points of interest arising from space formulation is the correspondence between configuration and a certain physical property, viz., that of optical activity, by virtue of which certain compounds possess the power of causing the rotation of polarized light in either a right-handed (dextro) or left- handed (laevo) direction. It is, in fact, by this character alone that the stereo-isomerism is in most cases revealed, since such isomer- ides, unlike ordinary isomerides, are alike in all other physical and chemical characters. In the light of Stereo-chemistry, optical activity 226 CHEMISTRY is shown to be associated with this intra- molecular asymmetry. Physics, Chemistry, and Biology herein find another common meeting ground, since it is to Physics that we look for the explanation of the mechanism of the connection between asymmetry and opti- cal activity ; while the chemical processes which go on in the living organism frequently result in the apparently direct production of optically active carbon compounds an achievement which some chemists believe to be an essential privilege of " vitalism." But laboratory syntheses also result in optically active compounds ; the lactic and tartaric acids, the 6-carbon-atom sugars, and hosts of other compounds have all been synthesized in their stereo-isomeric optically active forms. The main difference is that biochemical synthesis is directive in the sense of leading to the final production of the optically active compound, while laboratory synthesis is at present without such directive power the two possible configurations are produced simultaneously, and the final product is, there- fore, optically inactive by compensation. But such inactive compounds can be afterwards separated or resolved into their stereo-iso- merides by temporary combination with other active compounds of vital origin. With the CHEMISTRY OF CARBON 227 solution of the fundamental problem of con- trolling laboratory synthesis so as to suppress the production of the one or the other of the possible intra-molecular configurations, the temporary aid of the optically active vital compound would be dispensed with. There would then disappear another of the barriers which have been erected between living and dead organic matter. The asymmetry possible for a quadrivalent atom, such as carbon, is obviously conceivable in the case of other atoms. The hint given by carbon chemistry has been taken with all the seriousness which attaches to what the man of science knows to be a great truth. Other quadrivalent elements, such as tin, silicon, and sulphur; quinquevalent elements such as nitrogen and phosphorus, and, quite recently, the atoms of certain metals such as cobalt, chromium, platinum, and rhodium, have been made to form optically active stereo-isomerides. The reader will realize that this new and vast domain which is being opened up by many workers in many lands invests the atom of modern Chemistry with a reality so vivid that the correspondence between mental imagery and observed fact cannot be said to be surpassed in any of the purely deductive sciences. CHAPTER X THE PERIODIC CLASSIFICATION OF THE ELEMENTS CONCLUSION A SCIENCE which, in its purely materialistic aspects, claims for its subject matter the study of some eighty odd elements, and all the compounds capable of being formed by these elements, would be but a heterogeneous jum- ble of facts without guidance from general principles. It has only been possible within the compass of this volume to give the reader a glimpse here and there into some of these principles. The treatment has perforce been narrowly materialistic ; and yet it must not be concluded that the chemist takes only into consideration the matter of which the universe is composed. The energy associated with this matter its distribution during chemical change, the development or absorption of heat as concomitants of chemical transforma- tions, the production of electricity, of light, and, generally, the physical manifestations 228 PERIODIC CLASSIFICATION 229 resulting from this redistribution of energy are as much within the province of modern Chem- istry as is the natural history of the elements and their compounds. An introductory frag- ment only has been offered in the hope that a stimulus may be given to the desire for fuller and more specialized study. With reference to matter as such, it will be readily seen that any scheme which enables the whole body of elements to be grouped and classified according to their natural relation- ships must mark an advance towards the sys- tematization of our knowledge of the highest order of importance. That natural relation- ships exist among the members of chemical families, which possess certain characters in common, and which also show regular grada- tions of properties when considered in series arranged in the order of their atomic weights, has already been illustrated by reference to the halogens (p. 100). Any other family, non- metals or metals, might have been made to enforce the same lesson. But such classifica- tion is restricted ; a wider and more compre- hensive scheme which embraces all the natural groups or families of elements was first sug- gested by J. A. R. Newlands in 1864, and was elaborated and put upon a scientific basis by Mendeleeff and Lothar Meyer in 1869. Brief 230 CHEMISTRY | o consideration to this scheme may 2 ,| g foe given in this concluding chapter. If the elements are arranged in So co rl o the order of their atomic weights, ~ j| a remarkable recurrence will be a, noticed after a certain number of !> 3 i g elements have been passed through. ^ S I* Thus, omitting hydrogen which ^ stands in a unique position a g OT series of eight members must be <"! ^ arranged in order to bring out the fact that with the ninth there g .1 ,_, begins another series, in which the j$ I S5 chemical and physical properties