ON PHYSICO-CHEMICAL METHODS AND PROBLEMS. 3 



promises the solution of an important problem, namely, 

 the energy of growth. 



Further relations between chemical constitution and 

 physical properties were discovered by the new science, 

 physical or theoretical chemistry. 



Under this heading must be mentioned first of all 

 the connection discovered between optical asymmetry 

 (rotation of the plane of polarized light) and asymmetry 

 in chemical composition. At almost the same time 

 LE BEL and VAN'T HOFF discovered that all optically 

 active substances which in the non-crystalline state 

 rotate the plane of polarized light contain an asymmetric 

 carbon atom, the four valencies of which are connected 

 with four different radicles. If we imagine these four 

 valencies connected with the corners of a tetrahedron, 

 the four radicles may be grouped in two different ways 

 and be symmetrical. The development of this idea, 

 which constitutes the foundation of stereochemistry, has 

 been very fruitful. 



The doctrine of the asymmetrical carbon atom is 

 destined to play an important role in biological problems 

 also, for such essential constituents of protoplasm as the 

 proteins and many carbohydrates must, to correspond 

 with their optical activity, contain such an asymmetrical 

 carbon atom. 



The connection between physical changes in state and 

 chemical constitution was early indicated by the regu- 

 larity with which bodies of the aliphatic series affect 

 the boiling-point. More recently a connection between 

 color and the position of certain groups in the molecule 

 known as chromophores has been discovered. Similar 

 conditions exist in the case of fluorescence which is 



