B.— CHEMISTRY. 47 



DO inconsiderable achievement to have gained in so short a time a know- 

 ledge of many of the reactions and properties of the more simple complexes 

 of carbon in combination with oxygen, nitrogen, and other elements. 

 But much yet remains to be done before we can attack with any real 

 hope of success the problems which the chemistry of nature presents. 

 It is true that the knowledge already gained has led to the synthetic 

 preparation of quite a number of natural products, many of which are of 

 service in relation to human needs. Many of the alkaloids, colourin'' 

 matters like indigo and alizarine, camphor, and a large number of natural 

 products, have yielded the secrets of their structures and have been produced 

 by laboratory methods and, where necessary, on the factory scale. But the 

 synthesis of such compounds has not provided much insight into the 

 mechanism leading to their production in nature, and, indeed, the reason 

 for their occurrence in the plant is not understood. They are, moreover, 

 crystalline substances which either occur in the plant as such or are formed 

 by the hydrolytic fission of some more complex plant materials. Their 

 homogeneity is, therefore, not open to doubt, and their degradation into 

 known fragments and the rebuilding of these fragments into the original 

 substances, although by no means easy, is nevertheless comparatively 

 simple when the difficulties attending the investigation of more complex 

 natural products are taken into account. Even so, some of the simpler 

 type, for example, strychnine, still resist the attack of the chemist. 



The Electronic Theory. 



It is clear that our knowledge of the finer mechanism of reactions is 

 slight, and that, great as has been the advance made through the discovery 

 of van't Hofi, we are still at a loss to explain or predict the shades which 

 determine whether one particular type of reaction will be more, or less, 

 facile than another. The chief trouble seems to be that the electronic 

 theories, which are quite satisfactory in themselves, are not yet developed 

 so fully that they can include any quantitative statement relating to the 

 changes in the free energy of systems. Yet it is evident that any theory 

 of organic structure must conform to the modern physical conceptions of 

 matter. The principle of shared electrons is primarily justified by its 

 success in explaining the linking of atoms, i.e. valency, and by its successful 

 interpretation of the theory of co-ordination and ' onium ' salt formation. 

 The subsidiary hypothesis of electron displacement also provides a means 

 by which an explanation can be supplied to account for the ease of forma- 

 tion, stability, and general reactions of conjugated systems, thus placing 

 the hypothesis of Thiele on a sounder theoretical basis. 



Butadiene. 



CH.,=CH-CH=CHo 



H H H H 



II II II II 

 "C=C=C=C- 



II 11 



H H 



(Thiele.) (Electronic formula.) 



