TKANSACTIONS OF SECTION J5. 



819 



atotu itself is placed. If the four atoms or groups are all identical they will be 

 equally attracted by the carbon atom ; consequently they will be equidistant from 

 it, and the tetrahedron will be regular. If they are all ditlerent the force with 

 which each is attracted will be different ; they will arrange themselves at different 

 distances from the carbon atom ; and the tetrahedron will be irregular : it will 

 have no plane of S3'mmetry. Any cooipound of the formula CIIX'Y'Z' can there- 

 fore exist in two enantiomorphs, applying this term to the molecules themselves — 

 ia two non-superposable forms, each of which is the mirror image of the other ; 

 thus — 



Fig. 1. 



Fig. 3. 



( In these figures no attempt has been made to represent the tetrahedra as irre- 

 gular ; the opposite asymmetry is indicated merely by the opposite order of the four 

 attached atoms or groups. In realiry, however, they would be irregular. The 

 carbon atom itself is not shown.) 



If we consider any particular set of three atoms or groups — for example, 

 II. Z', and Y' — looking towards that face of the tetrahedron about which they are 

 arranged, any order, thus HZ' Y', which is clockwise in one figure, will be counter- 

 clockwise in the other. In like manner, a continuous curve, passing through the 

 four atoms or groups in any given sequence, will form a right-handed helix in the 

 one case and a left-handed helix in the other. We thus find that the foregoing 

 assumptions — the very simplest that could be made — regarding the distribution of 

 the four affinities of carbon and the different degree with which four difierent 

 atoms or groups will be attracted by the carbon atom to which they are attached, 

 lead to the asymmetric structures postulated by Pasteur to account for optical 

 activity — namely, enantiomorphous irregular tetrahedra, and right- and left-handed 

 helices. 



M ^' 



^H 



r 



That a spiral arrangement, right- or left-handed, will produce rotation of the 

 plane of polarisation in its own sense, may be shown by various experiments: 

 thus in Keusch's optically active piles of plates of mica, produced by crossing 

 successive plates of biaxal mica at an .angle of 60° to one another ; or in the twisted 

 jute fibres recently described by Professor Bose, which, according to the direction 



3 G 2 



y 



■^ n 



6U> 



