138 SCIENCE PROGRESS. 



matical certainty the evidence in favour of the enantio- 

 morphic formulae assigned to the two isomers. 



Various models have been devised to illustrate the 

 tetrahedral configurations, and it cannot be too strongly 

 unjed that models of some kind are essential to a clear 

 understanding of the subject. Van't Hoff used cardboard 

 tetrahedra, with corner-caps differently coloured to re- 

 present the different groups. By placing four caps of 

 different colours, in different order on each of two tetrahedra, 

 we obtain the two asymmetric configurations related as 

 object and image, representing the two isomers CR'R'R' "R 1V . 

 But if we make two of the corner-caps of the same colour 

 on each tetrahedron the two configurations become identical, 

 and each possesses now a plane of symmetry. The same 

 thing may be represented by the Kekule models as modified 

 by v. Baeyer. 



A simple and inexpensive form of model has been de- 

 vised by P. Friedlander, 1 and consists of four short pieces 

 of caoutchouc tubing, symmetrically diverging from a centre 

 where they are joined together (according- to Van't Hoff 

 with sealing-wax). 



I am indebted to Mr. H. A. D. Jowett for a description 

 of similar models, which any one may readily make for him- 

 self by attaching short rubber tubes plugged with cork 

 to four equidistant points on a small, hollow rubber ball by 

 means of tacks (head inside the ball) and rubber cement. 

 This arrangement is of advantage if one wishes to represent 

 the carbon atom as having an appreciable size compared 

 with the distance between the atoms. Moreover, these 

 models are more easily made of the requisite symmetry 

 than are Friedlander's. 



Another method is to solder together at the ends two 

 brass rods, each bent at the middle at an angle of 109 ; 

 coloured rods, or balls, or flags bearing formulae may be 

 attached to the ends of the brass rods by means of rubber 

 connectors. 



While for private study it is desirable to use models as 

 small as can be conveniently handled, for lecture purposes 



1 Ber. d. Chem. Ges., xxiii., 572. 



