148 



THE CARBOHYDRATES. 



[CH. V. 



The accepted explanation of the facts is that the carbon 

 atom possesses four valencies or bonds directed towards 

 the apices of a tetrahedron, the carbon atom being at the 

 centre. 



The student is advised to construct such a model from 

 a piece of plasticine and four matches, the central piece of 

 plasticine representing the carbon atom and the matches 

 the four bonds attached to it. The heads of the matches 

 should be left plain or marked with little balls of variously 

 coloured plasticine to indicate the different groups attached 

 to the central carbon atom. Such a model is represented 

 in fig. 24. 



A 



Fig. 24. Model of carbon atom attached to 3 different groups, representing 

 the compound Ca z bx. 



Another identical model should now be prepared. It 

 will be found that the two can be superposed, as shewn 

 in fig. 25. 



Now change the positions of any two matches in one 

 of the models. It will be found that the two models can 



