MOLECULAR STRUCTURE AND LIFE — PIOTET. 201 



what is strictly necessary. It will be sufficient for the purpose of 

 my demonstration to bring to your attention the principle of organic 

 classification. 



As the result of 50 years of patient researches it has been ascer- 

 tained that the approximately 200,000 organic compounds now 

 known, however great their diversity, belong, from the viewpoint 

 of their molecular structure, to only two types. 



In the first type, the atoms of which they are formed, whether 

 saturated or unsaturated compomids, are joined in a nearly recti- 

 linear chain of greater or less length. The central part of the mole- 

 cule forms a sort of vertebral column to which in turn other atomic 

 groups are joined laterally. 



In the second type these same atoms are joined under the influence 

 of similar attractive forces, but form closed chains. The structure 

 of the molecule is now not a string of atoms but a ring. And on this 

 ring similar circular groups are applied just as the tissues of a fruit 

 are built up on its stone or kernel. 



Hence we have the distinction between compounds with open 

 chains and those with cyclic radicals. This distinction lies at the 

 very foundation of organic classification. It corresponds, for ex- 

 ample, to what in zoology is the division between vertebrates and 

 invertebrates, and is not without analogy to it, for it is founded on 

 the conformation of the structure and on the symmetry of the being, 

 whether it be an animal or a molecule. 



From a theoretic point of view the two great classes of organic 

 compounds are separated by a great gap. But this is not insupera- 

 ble. In many cases, by suitable reactions, it is possible to act on the 

 molecules of substances in such a way as to close an open chain 

 (cyclisation) or to break a closed chain (cyclolyse). Thus it is possi- 

 ble to pass experimentally from one type to the other. 



It is true that this transition is incomparably easier in one case 

 than in the other. One of the characteristics of the closed chains is 

 their stability, considerable chemical energy being always required 

 to disorganize them. On the other hand, cyclisation is more easily 

 effected, although it demands a certain degree of energy, required 

 for the bending of the rectilinear chain and the welding of its ter- 

 minal atoms. What are the forms of energy needed to produce this 

 result? 



In the first place is heat. Berthelot first showed this by passing 

 through red-hot tubes an. entire series of open-chain substances. 

 He thus obtained numerous cyclic compounds, and in particular the 

 greater part of those that in combination constitute coal tar, a by- 

 product of gas manufacture fiom which the modem chemist has ob- 

 tained so many valuable derivatives. On the basis of these experi- 

 ments, Berthelot likewise founded his well-known theory of the 



