PRESENT PROBLEMS 291 



from my main purpose. Before leaving this topic I must add, how- 

 ever, that I have used the phrase "Avogadro's law" advisedly, in 

 spite of the fashion set by some chemists of calling it Avogadro's 

 hypothesis. 1 



I remarked, a few moments ago, that the facts which have been out- 

 lined almost compel us to the acceptance of van't Hoff 's hypothesis 

 in some form. It is of the utmost importance for us to recognize, 

 however, that we are here at the very confines of our present know- 

 ledge, and that we must, at every step, bring ourselves back to the 

 rigorous test of experimental fact. In accepting the hypothesis we 

 are not compelled to consider molecules as set pieces of mechanism; 

 on the contrary, there is strong reason for thinking that the positions 

 assumed by the atoms are positions of dynamic and not of static 

 equilibrium. While there have been many speculations in the matter, 

 we have no strong reason for assuming, as yet, any definite shape 

 for the carbon atom, nor even that there are within it definite points 

 of attraction for other atoms. All that seems to be thoroughly estab- 

 lished is that for their position of most stable equilibrium the four 

 atoms or groups attached to a given carbon atom are arranged in 

 approximate symmetry around its centre. I say approximate sym- 

 metry because the existence of compounds containing rings of three 

 and four carbon atoms demonstrates that the symmetry is not 

 always absolute, and makes it probable that in cases where the four 

 atoms or groups are unlike the symmetry is also imperfect. So far as 

 I am aware, no fact inconsistent with this fundamental conception 

 is known, while very many facts about optically active and cyclic 

 compounds find in this conception the only satisfactory explanation 

 which has thus far been given. It is true, also, that many facts with 

 regard to optically active compounds indicate that when one group 

 is exchanged for another the exact configuration is often retained, 

 or, in other words, the entering group takes the same position with 



has been determined, the composition of a series of compounds between A and C 

 can be predicted and a compound which does not belong to this series has never 

 been discovered. A still more general statement of the law, and one which includes, 

 by implication, all of those facts which are used in the selection of atomic weights, 

 is given above. In that form it is more properly called the law of atomic weights. 

 1 Two reasons may be given for this usage. My own view is that we have, by a 

 process of inductive reasoning, acquired such positive knowledge of the existence 

 of atoms and molecules that the expression " Avogadro's law " is fully justified. 

 But even if we admit the contention of those who think that the atomic theory 

 must always remain an unproved hypothesis, it is possible to frame a definition 

 of the word molecule which would be merely a generalized statement of those 

 empirical facts which lie at the basis of our atomic and molecular theories. Such 

 a generalized, empirical definition must, of course, be very complex, but it would 

 not include the concept of discrete particles. Yet it will be still true of these 

 empirically defined molecules that equal volumes of gases contain equal numbers 

 under the same conditions of temperature and pressure. For instance, the term 

 gram-molecule may be considered as a purely empirical generalization, and it is 

 true that a gram-molecule of one gas occupies the same volume as a gram-mole- 

 cule of any other. But this is, in essence, Avogadro's law. 



