SYNTHETIC WORK IN CARBOHYDRATES 325 
This theory explains the existence of two or more compounds 
of like chemical composition, by assuming different disposi¬ 
tions of the atoms entering into the compound. 
The simplest hydrocarbon, methane, is conceived as being 
a tetrahedron with a carbon atom in its centre and one hy¬ 
drogen atom joined at each of its four angles. The carbon 
atom of this compound is symmetrical, inasmuch as all the 
atoms to which it is united are of a like kind. In such a case 
stereomers are impossible. 
But in order to have the conditions for stereosomerism, it 
is necessary for a compound to contain one or more atoms of 
asymmetrical carbon; that is, a carbon atom united by all of 
its four bonds to atoms or groups of atoms of different kinds. 
Methane may be represented, for illustration, by a paste¬ 
board tetrahedron model, 1 the angles being painted red to 
distinguish the points of carbon’s union with hydrogen atoms. 
If this model be placed angle to angle with a second methane 
tetrahedron, the hydrogen atoms will coincide, and if one of 
the models be superimposed upon the other, the hydrogen 
atoms at each of the angles will touch, showing the symme¬ 
trical grouping. The symmetry of the molecule is not dis¬ 
turbed when two or three different kinds of atoms replace the 
hydrogen atoms of methane. But when all of the hydrogen 
atoms are replaced by different kinds of atoms, it will be found, 
on bringing the angles of like color of two models together 
and superimposing the one model upon the second model, 
that the angles of like colors cannot be made to coincide. 
Lactic acid is an illustration of a compound containing an 
asymmetrical carbon. This compound, represented by the 
constitution CH 3 . CHOH . CO OH, contains two symmetri¬ 
cal carbon atoms, one at either end; the carbon atom which 
occupies the middle position is the asymmetrical carbon, since 
this atom is united by its four bonds with different atoms or 
groups. The presence of this middle carbon atom induces 
the conditions which cause lactic acid to appear under two 
acid modifications. By the action of these compounds on the 
1 The subject here and what follows was explained by means of models 
and charts. 
