Februart 21, 1919] 



SCIENCE 



177 



compound, Sn(C.,H3).,0, while an atom of 

 tin alone will combine -with two atoms of 

 oxygen to form stannic oxide, SnO^- 



A few years later Couper and Kekule, quite 

 independently of each other, developed clearly 

 the idea that carbon compounds are held to- 

 gether in chains by attractions between the 

 atoms and tliat the structure of the molecules 

 of such compounds is directly dependent on 

 the valence of the atoms of which they are 

 composed. 



In the same year, 1858, at Genoa, Cannizzaro 

 revived the hypothesis of Avogadro and Am- 

 pere and gave such convincing evidence of 

 its truth that it was soon accepted by the lead- 

 ing chemists of the world. This introduction 

 of a correct system of atomic and molecular 

 weights aided greatly in the very rapid de- 

 velopment of structural organic chemistry. 

 We can still imagine with what enthusiasm 

 the chemists of that day seized the key to 

 nature's mysteries which the doctrines of 

 valence and of the linking of atoms had given 

 them and applied them to the solution of 

 problems of structure and of synthesis. Only 

 a few years before the thought of definite, ac- 

 curate knowledge of this kind would have 

 seemed the dream of a hair-brained visionary. 



Chemistry is primarily an experimental sci- 

 ence. New theories make their way slowly and 

 speculations which are not forced upon us by 

 incontrovertible facts have met with little 

 favor. At the time when the theory of 

 valence made itself indisi>ensable as a guide 

 to the investigation of carbon comiwunds the 

 older electrochemical theory had practically 

 disappeared and no theory for the cause of 

 the attraction between atoms received more 

 than passing attention. It was tacitly as- 

 sumed that some sort of attraction between 

 atoms held them bound together but even 

 such necessary terms as " single bonds " and 

 " double bonds " or " linkages " were used 

 with reserve by many chemists. 



During the forty years following the pub- 

 lication of the papers by Coupor and Kekule 

 the theory was amplified in only one impor- 

 tant detail. The original theory considered 

 only the sequence of atoms in compounds. 



While there may have been occasional thoughts 

 about arrangements in space, chemists were 

 very reticent in expressing them. In 1874, 

 however, van't Hoff proposed an explanation 

 of the relation between the structure of op- 

 tically active compounds and the arrangement 

 of their atoms, based on the fundamental 

 proposition that four univalent atoms or 

 groups combined with a given carbon atom 

 are arranged symmetrically about the center 

 of the atom. From the same starting point 

 he postulated the supposition that two carbon 

 atoms connected by a double union can not 

 rotate independently about the jwints of 

 union. The first hypothesis gave a satisfac- 

 tory explanation for optically active com- 

 pounds and it is impossible now for any one 

 to question the fact that a compound which is 

 optically active in solution must contain a 

 central atom or group around which four or 

 more atoms or groups are arranged in an 

 asymmetric fashion. Incidentally it may be 

 remarked that the discovery of compounds in 

 which the asymmetric atom is nitrogen or 

 sulfur or tin demonstrates that the principle 

 of valence is general in its application and is 

 not simply of value for carbon compounds. 



The use of van't Hoff's principle in the 

 explanation of the isomerism of such com- 

 pounds as fumaric and maleic acids was 

 equally successful. 



In 1885 Bayer gave the following statement 

 of the well-established principles used in ex- 

 plaining the structure of carbon compounds' 



1. Carbon is usually quadrivalent, 



2. The four valences are alike. 



3. The valences are symmetrically directed 

 in space from the center of the carbon atom. 



4. Atoms attached to the four valences do 

 not easily exchange places — van't Hoff's prin- 

 ciple. 



5. Carbon atoms may be united with one, 

 two or three valences. 



6. The compovmds may be either open 

 chains or rings. 



Baeyer proposed a seventh principle: 



7. The directions in which the valences are 

 exerted may be diverted from the normal 

 angle of the tetrahedron, which is 109° 28', 



