SPACE-ISOMERISM 



79 



instead of drawing the actual tetrahedral perspective, to 

 write a projection of the formula on the plane of the paper 

 (Kg. 21), the asymmetric carbon atom being distinguished by 

 a circle rotmd it or by heavy type. Thus the dextro and 

 leevo lactic acid can be written as follows :— 



CH, 



CH, 



COjH CO2H 

 Fig. 21. 



or in projection looked at from above : — • 



CH, 



CH, 



OH 



HOCH 



COjH 



HCOH 

 CO,H 



The case of the tartaric acids is somewhat more complex, as 

 there are here two asymmetric carbon atoms in the molecules; 

 the following configurations are therefore possible : — 



