A HUNDRED YEARS OF CHEMISTRY. 681 



In 1858 the work of Cannizzaro appeared, and a general revi- 

 sion of chemical formulae became necessary. The advanced views 

 which a few chemists had held began to find a more general accept- 

 ance, and the significance of the change was gradually realized. 

 In the same year Kekule showed that the atom of carbon, had a 

 combining capacity of four, and furthermore that in many organic 

 compounds the carbon atoms were in part united with each other, 

 and even linked, as it were, into chains. Still later, studying ben- 

 zene, he found that its six carbon atoms were best regarded as 

 joined together in the form of a closed ring, and with this concep- 

 tion the idea of chemical structure received at last a definite form. 

 These linkages of atoms, these rings and their derivatives, could 

 all be represented graphically to the eye, in accordance with the 

 combining power of the several elements, and so the structural 

 formulae of modern chemistry came into vogue. Types, substi- 

 tutions, compound radicles, were all covered by and included in 

 the new generalization, and each of the older theories was seen 

 to be but an expression of special cases, rather than of any gen- 

 eral law. ISTo truth was set aside, but all were co-ordinated. 



To the non-chemical reader the foregoing passages may seem 

 vague and abstruse, but in an essay of this scope greater elabora- 

 tion is inadmissible. It is clear, however, that each forward step 

 has been a logical development of the atomic theory, which, as we 

 shall see later, does not end even here. 



Thus, then, the chemical formulae and atomic weights of Ber- 

 zelius grew by slow degrees into the modern system, with its repre- 

 sentations of structure and atomic linking. The internal archi- 

 tecture of the molecule was now revealed not to the imagination 

 only, but to the eye of reason, and, speculative as the new concep- 

 tions may seem at first, they have led to astonishing practical con- 

 sequences. The new formulae at once indicated lines of research, 

 and with their aid synthetic chemistry was greatly stimulated. 

 True, many syntheses of organic compounds had already been 

 made, but progress became more rapid and the work of discovery 

 was systematized to a wonderful degree. In 185G Perkin discov- 

 ered the first of the coal-tar dyes, creating a new industry which 

 has been assisted beyond measure by the structural symbols that 

 came into use only a few years later. In 1868 alizarin, the color- 

 ing principle of madder, was made artificially from the hydrocarbon 

 anthracene; a host of other colors, a veritable chemical rainbow, 

 have been discovered; the synthesis of indigo has been effected; 

 and within twenty years we have seen medicine enriched by a great 

 variety of drugs, all prepared by purely chemical processes from 

 the former waste material — coal tar. To most of this work, at 



