4 88 SCIENCE PROGRESS 



in such a way that the plane passing through the two arms of 

 the one is at right angles to that passing through the two arms 

 of the other. The four arms should be made of equal length. 

 Such model carbon atoms can be united by juxtaposing an 

 arm of each of two skeleton tetrahedra and then binding these 

 together. Provided they be arranged in spiral order any desired 

 number may be united in such a manner, but the number 

 which can be arranged so that their centres lie in one plane 

 is strictly limited : five naturally form a ring which is practically 

 complete; any less number gives an incomplete ring: the significance 

 of this peculiarity will be discussed later on. Models of the 

 higher paraffins made with such skeleton tetrahedra present the 

 appearance of curls rather than of straight chains. 



The adoption of this tetrahedral conception of the functional 

 activity of a carbon atom is justified in the most absolute manner 

 possible by the discovery which is so indelibly associated with 

 the name and fame of the great Pasteur — namely, the discovery 

 of two modifications of tartaric acid identical in composition, 

 which are distinguishable only by a minute difference in crystal- 

 line form and by their behaviour in polarised light : a solution 

 of the one having the power of deflecting a beam of polarised 

 light just as far to the right as an equally concentrated solution 

 of the other will to the left. In tartaric acid 



CH(OH)(COOH) 

 CH (OH) (COOH) 



each of two carbon atoms — those printed in clarendon type — is 

 associated with four different radicles H, (OH), (COOH) and 

 [CH(OH)(COOH)]. 



If two large cardboard tetrahedra be taken, the one without 

 one of its sides so that it can be superposed upon the other, it is 

 easy to fix four differently coloured caps over the four solid 

 angles of each tetrahedron so that when the two tetrahedra 

 are superposed like colours occupy like positions on each. But 

 take off" the capping tetrahedron and place the other in front 

 of a mirror or looking-glass : note the position of the coloured 

 angles in the image and assume that the tetrahedron used as 

 cap is in the position of the image, then arrange the colours 

 at its angles so that they are in the position which they 

 occupy in the image ; superpose this capping tetrahedron on 



