PRESENT PROBLEMS 289 



arrangement of atoms in space. So far as the original and funda- 

 mental conceptions are concerned, however, this knowledge is quite 

 vague. The much more definite conception proposed by van't Hoff , 

 and in a somewhat different manner by Le Bel, is, of course, familiar 

 to you all. In discussing any hypothesis it is always important to 

 have clearly before us the facts upon which it is based. As I have 

 already hinted, I believe that the theory of valence and the theory 

 of structure in the sense of a sequence of atoms within the molecule 

 are supported by our knowledge of such a vast accumulation of con- 

 sistently interrelated phenomena that we are justified in believing 

 that we have positive knowledge with regard to the structure of the 

 molecules of organic compounds. I am as ready as any one to de- 

 mand that every theory, no matter how old or how universally . 

 accepted, shall be continually brought back to the test of agreement 

 with experimental facts, but I am not willing to admit that we may 

 not, in the end, acquire positive knowledge by the process of inductive 

 reasoning. 



Assuming, then, the fact of a knowledge of the sequence of atoms 

 in organic compounds, we have this basis for van't Hoff's hypothesis: 

 (1) When four unlike atoms or groups are combined with a single 

 carbon atom, optical activity results in such a manner that there may 

 always be found two compounds having identical sequence of the 

 atoms within the molecule, and exactly equal rotary power, but of 

 opposite signs. (2) That when two adjacent carbon atoms are com- 

 bined each with three unlike groups, two compounds may result 

 which, while optically inactive and having the same sequence of 

 atoms, still differ in physical properties. An illustration of this is 

 found in racemic and mesotartaric acids. (3) Rings containing five 

 and six atoms are formed with especial ease, those containing three, 

 four, and seven atoms less readily, and rings containing more than 

 seven atoms are scarcely known. (4) Derivatives of cyclopropane, 

 cyclobutane, cyclopentane, and cyclohexane having two substituents 

 combined with different carbon atoms often exist in two isomeric 

 forms in which the sequence of the atoms is the same. (5) Deriva- 

 tives of ethylene often exhibit a similar isomerism. 



Assuming as true that we have acquired a knowledge of the sequence 

 of atoms in carbon compounds, the facts which I have enumerated 

 lead almost inevitably to the corollary that the four atoms attached 

 to a given carbon atom are arranged in approximate symmetry 

 around the centre of that atom for their position of most stable 

 equilibrium. The relation between this conclusion and the theory of 

 the sequence of atoms in carbon compounds, or what is ordinarily 

 understood as structure, is very similar to the relation between the 

 atomic theory and Avogadro's law. If we accept the atomic theory, 

 there seems to be no rational escape from the acceptance of Avogadro's 



