216 CHEMICAL DISCOVERY AND INVENTION 



regarded as a compound of the other two. But it contains 

 within the molecule two asymmetric carbon atoms indicated by 

 black type in the formula, 



H 

 HO C CO-OH 



I 

 HO-C CO-OH 



H 



and the action of one of these on the polarised ray may be sup- 

 posed to be equal and opposite to the action of the other, so that 

 the effect is the same as if they existed in separate molecules, 

 mixed together in exactly equal numbers, as in the case of 

 racemic acid. 



Van 't Hoffs hypothesis, which serves to explain these facts, 

 supposes the carbon atom to be situated at the centre of a 

 regular tetrahedron, while the four other atoms united with it 

 are situated at the solid angles ; so that the four valencies of the 

 carbon atom are supposed to operate in the directions of four 

 radii of a sphere included in the tetrahedron or which includes it. 

 Suppose the atoms united with a carbon atom to be repre- 

 sented by letters, then 

 when one atom of car- 

 bon is united with 4A, 

 with 3A+1B, with 

 2 A + 2 B or with 

 2A+1B+1C isomer- 

 ism is impossible, that 

 is there can exist only 

 one compound of this constitution. But when all four of the 

 attached atoms or groups of atoms, A, B, C, D, are different two 

 cases occur. These may be represented either by a tetra- 

 hedron, the angles of which are lettered as in figure 59; or 

 by a conventional symbol, which is more easily printed thus 

 a c b boa 



a 





d 



