28o 



NATURE 



[July 23, 1903 



The German Royal Naval Observatory. — The twenty- 

 fifth annual volume (1902) of the publications of this 

 observatory, entitled " Aus dem Archiv *der Deutschen See- 

 warte, " contains descriptive papers on "The Regulation 

 of Marine Compasses," "A New Free-horizon Astro- 

 nomical Base Line," " The Definitive Determination of the 

 Path of the Comet Swift (1899.I)," and "The Results of 

 Sextant Tests made at the Observatory. 



In addition to the introduction, Dr. Neumayer, the 

 director, contributes an article on " A New Method of Fore- 

 casting the Meteorological Conditions of the North Atlantic 

 Ocean," and a novel chart, indicating all the meteorological 

 conditions obtaining in the North Atlantic area during 

 March, 1902, accompanies the volume. 



RECENT ADVANCES IN STEREOCHEMISTRY.^ 

 TN the year 1803, just a century ago, John Dalton de- 

 -"- livered a series of scientific lectures in the Royal In- 

 stitution during the course of which he doubtless laid 

 before his audience a theory which he had recently devised 

 for the purpose of connecting together the vast number of 

 isolated chemical facts known at the commencement of the 

 nineteenth century. This theory, of which the centenary 

 is being celebrated during the present month by the Man- 

 chester Literary and Philosophical Society, is known as 

 the atomic theory, and was destined to form the foundation 

 upon which the whole superstructure of modern chemistry 

 has been built. For our present purpose Dalton 's theory 

 may be briefly stated in the form of the following two 

 principles : — (i) Every element is made up of homogeneous 

 atoms of which the mass is constant ; (2) chemical com- 

 pounds are formed by the union of atoms of the various 

 elements in simple numerical proportions. In accordance 

 with Dalton 's hypothesis, chemical substances may be 

 mentally pictured by imagining the atoms as small spheres 

 which have the power of aggregating themselves together 

 under suitable conditions to form complexes or " mole- 

 cules " ; thus, taking two similar spheres representing 

 hydrogen atoms, in conjunction with a sphere of a different 

 kind, representative of an atom of oxygen, a chemical re- 

 presentation can be given of the compound water, the 

 molecule of which is composed of two atoms of hydrogen 

 and one of oxygen. The original atomic theory offers no 

 explanation of the observed fact that the atoms combine 

 together in different proportions ; this deficiency was 

 remedied by the doctrine of valency enunciated by the late 

 Sir Edward Frankland in 1852. Frankland supposed that 

 the atoms of certain elements, such as hydrogen and 

 chlorine, are unable to combine with more than one atom 

 of any other element ; these elements are termed mono- 

 valent. Other atoms, such as those of barium and zinc, 

 can become directly attached to at most two other atoms ; 

 these are the divalent elements. Tri-, tetra-, penta-, hexa-, 

 hepta- and octa-valent elements' can be similarly dis- 

 tinguished, the valency of hydrogen being taken as unity, 

 in order to measure and define the saturation-capacity or 

 the atom-fixing power of the atoms of the other elements. 

 It will be clear that for rough diagrammatic purposes we 

 may provide the spheres representing the atoms with as 

 many wooden pegs as the element itself exhibits units of 

 valency ; compound molecules can then be represented by 

 fitting the atoms together by means of the pegs represent- 

 ing the number of valency-units possessed by the various 

 constituent atoms. By so doing a great advance is made 

 upon the atomic theory of Dalton 's time, and a mental 

 picture is obtained of the way in which the atoms are con- 

 nected together within the molecule itself. 



During the early part of the nineteenth century it became 

 evident, principally from the work of Liebig and Wohler 

 in Germany, and of Faraday at the Royal Institution, that 

 substances exist which possess totally different properties, 

 but nevertheless have the same molecular composition ; as 

 this became slowly realised, the atomic theory was naturally 

 called upon to furnish some adequate explanation. In view 

 of the proven identity of molecular composition, the re- 

 quired explanation could only be sought for in differences 



1 A discourse delivered at the Royal Institution on May i by Proi 

 William J. Pope, F.R.S. 



in the atomic arrangement within the molecules of the 

 several substances. That such differences can be success- 

 fully illustrated by the aid of the atomic models will br 

 seen on considering some specific case. Ordinary ethyl, 

 alcohol and methyl ether differ greatly from each other — 

 the first is a liquid, whilst the second is a gas at ordinary 

 temperatures— but possess the same molecular composition, 

 the molecule in each case consisting of two atoms of carbon, 

 six of hydrogen and one of oxygen. These two substances 

 have to be represented on the assumption that hydrogen 

 is monovalent, carbon tetravalent, and oxygen divalent. 

 By joining wooden spheres together in the order shown in 

 the figures — in which the valencies of the component atoms 

 are carefully respected — diagrammatic representations are 

 obtained which illustrate io the chemist the differences exist- 

 ing between ethyl alcohol and methyl ether. 



H H 



H 



NO. 1760, VOL. 68] 



H— C— C-OH H— C-O— C-H 



II II 



H H H H 



Ethyl Alcohol. Methyl Ether. 



Substances related to each other in this way are said to be 

 isomeric ; they have the same molecular composition, but 

 different molecular constitutions. The step in advance- 

 which is involved in thus writing molecular constitutions or 

 in constructing molecular models was taken by Kekul6 ins 

 1858. 



Two great stages in the development of chemical theory 

 have now been indicated. First, that contributed by 

 Dalton, who regarded constancy of molecular composition 

 as characteristic of a chemical substance ; secondly, that 

 further stage, attained as a result of the labours of Liebig, 

 Wohler, Faraday, Frankland and Kekul6, which involved 

 the introductipn of the idea that the chemical individuality 

 of a substance is dependent upon its molecular constitution, 

 as well as upon its molecular composition. A third great, 

 development in the atomic theory had yet to take place. 



Whilst the theoretical views which culminated in Kekul6's- 

 constitutional formulae were at first found sufficient to ex- 

 plain numerous observed cases of isomerism, instances soon- 

 began to accumulate of substances which exist in so many 

 isomeric forms that the Kekuld method of representation, 

 is incapable of accounting for them all. At an early date 

 Pasteur showed clearly that substances exist which have 

 the same molecular composition and the same molecular- 

 constitution, but which nevertheless differ in important 

 lespects. A crisis was ultimately reached when, in 1870,, 

 Wislicenus demonstrated the existence of three isomeric 

 lactic acids, all having the molecular composition C3H5O3.. 

 and the molecular constitution 



CH, 



OH 

 I 

 -C— COOH 



H 



and contended that he had amply proved the insufficiency, 

 of Kekul^'s method of writing constitutional formulae. 



The step needed to rid the atomic theory of these apparent 

 anomalies was indicated by van 't Hoff and Le Bel in iSjd ; 

 they pointed out that the weakness of the Kekul^ method 

 lies in the tacit assumption that the molecule is spread out 

 upon a plane surface, and that by throwing this assump- 

 tion aside and taking a rational view of the way in which 

 the molecule is extended in space, all difficulties immedi- 

 ately vanish. The considerations put forward by van 't 

 Hoff and Le Bel form the basis of the subject now known 

 as stereochemistry, the branch of science which deals with 

 the manner in which the atoms are distributed within the 

 molecule in three-dimensional space ; they deal, in the first 

 place, with the arrangement of the constituent atoms in 

 the simple organic compound, methane, the molecule of 

 which has the composition CH^, or consists of one carbon 

 atom and four hydrogen atoms. The Kekul6 constitutionar 

 formula pictures the component atoms of the methane mole- 

 cule as if joined together in one plane (Fig. i), whilst: 

 according to the new view, the four hydrogen atoms are 

 imagined situated at the four apices of a regular tetra-r 



