July 23, 1903] 



NATURE 



281 



4ieclron of which the carbon atom occupies the centre (Fig. 

 a). This is conveniently illustrated with the aid of a few 

 -ardboard models. 





To illustrate this we may refer to a somewhat complicated 

 substance, termed tetrahydroquinaldine, which has the 

 following constitution : — 



H Hj 



I I 



C C 



H-C 



-C C 



c 



I 



H 



CH2 



CH3 



Consider now the result of repricing three of the four 

 lydrogen atoms present in the mwhane molecule by three 

 itTerent groups of atoms, the three groups CH,, OH, and 

 t OjH for example. One of the most striking results 

 ■which has accrued from the chemical investigation of the 

 past century has been the demonstration of the remarkable 

 rigidity with which the atoms are held together in the 

 molecule ; it might therefore be anticipated that by actually 

 making all the isomerides having the constitution indicated 

 -above, some means would be afforded of judging whether 

 the van 't Hoff-Le Bel or the Kekul^ view forms the closest 

 approximation to truth. Kekul^'s constitutional formulae 

 indicate the existence of two isomeric compounds of the 

 following types : — 



H OH HO H 



\^ \/ 



C and C 



and the molecule of which contains an asymmetric carbon 

 atom, that, namely, which is printed in heavy type. Three 



CH, 



COOH 



CH3 COOH, 



•whilst on the van 't Hoff-Le Bel view, two isomerides of 

 the nature illustrated by Figs. 3 and 4 are indicated ; 

 although in each case two isomerides would be obtainable, 

 •the examination of the two kinds of figure reveals very 

 -essential differences. The solid-figure isomerides differ only 

 in that the one is the image in a mirror of the other — 

 <they are related in the same kind of way as a right 

 and a left hand glove. The differences observable between 



two molecules thus related should con- 

 •sequently not be differences of an ordinary 

 •chemical nature, but differences involving 



merely a kind of chemical, physical and 



mechanical right- and left-handedness. 



The two Kekul6 constitutional formulae, 



on the other hand, would indicate — if they 

 indicate anything — that the substances to 



which they refer differ in the more gross 



way in which ordinary chemical iso- 

 merides differ in chemical, physical and 



mechanical respect. That carbon atom 



Avhich was present in the original methane I 



molecule is, in these new compounds, 



-now attached to four different atomic groups, and such a 



carbon atom is termed an " asymmetric " carbon atom. 



It is in the case of substances containing an asymmetric 



; arbon atom that a lack of agreement is observed between 

 the facts and the kind of isomerism indicated by the Kekul^ 



CHi. 



Fig. 6. 



Fig. 



iormulae, and in these cases, also, the species of isomerism 

 indicated by the solid models exhibited is found to corre- 

 ■epond closely with the facts. 



NO. 1760, VOL. 68] 



different isomeric forms of this substance exist, and are quite 

 indistinguishable by any of the ordinary methods of chemical 

 or physical identification ; one of these is a loose kind of 

 compound of the other two, and may therefore be dis- 

 regarded for the moment. The remaining two have the 

 same melting point, the same boiling point, and correspond 

 exactly in all ordinary properties ; they yield, however, 

 series of derivatives which differ in the same sort of way 

 that a right-hand and a left-hand glove differ. Here, for 

 instance, is a diagram showing the shapes of the crystals 



Fig. 7. Fig. S. 



of the salts which these two substances form with hydro- 

 chloric acid (Figs. 5 and 6) ; the crystals obtained from the 

 one base are the mirror-images of those prepared from the 

 other. Any figure which possesses handedness of the kind 

 exhibited by these two crystal figures is termed " enantio- 

 morphous," and two figures which are related to each other 

 as these figures are related are said to be " enantio- 

 morphously related." A hand is thus enantiomorphous, 

 and a right and a left hand are enantiomorphously related, 

 the one being the mirror-image of the other. Here, for 

 example, is a photograph showing a right hand and a left 

 hand side by side (Fig. 7) ; the pair of hands is exactly 

 reproduced in the next photograph (Fig. 8), which shows 

 a right hand side by side with the photograph of its reflec- 

 tion in a mirror. Just the same enantiomorphous relation- 

 ship as that existing between the right and the left hand, 

 exists between the molecular pictures of the two lactic acids 

 discovered by Wislicenus, and shown in Figs. 3 and 4. 



Reference may now be made to the existence of other 

 differences of an enantiomorphous character between sub- 

 stances which possess enantiomorphously related structures. 

 Early in the last century the French physicists Arago and 

 Biot showed that a number of substances have the power 

 of deflecting the plane of polarisation of a plane-polarised 



Fig. 



