298 



SCIENCE 



[N. S. Vol. XXX. No. 766 



theory of the distribution of energy in the 

 spectrum of a hot body, Planck found that 

 e =: 4.69 X 10-", and N = 2.80 X 10'". 

 For reasons that we can not enter into here, 

 this theoretical deduction must be given 

 great weight. 



When we consider the great diversity of 

 the theories and methods which have been 

 utilized to determine the values of the 

 atomic constants e and N, and the probable 

 experimental errors, the agreement among 

 the numbers is remarkably close. This is 

 especially the case in considering the more 

 recent measurements by very different 

 methods, which are far more reliable than 

 the older estimates. It is difficult to iix on 

 one determination as more deserving of 

 confidence than another ; but I may be par- 

 doned if I place some reliance on the 

 radioactive method previously discussed, 

 which depends on the charge carried by the 

 o particle. The value obtained in this way 

 is not only in close agreement with the 

 theoretical estimate of Planck, but is in 

 fair agreement with the recent determina- 

 tions by several other distinct methods. 

 We may consequently conclude that the 

 number of molecules in a cubic centimeter 

 of any gas at standard pressure and tem- 

 perature is about 2.77 X 10'^ and that the 

 value of the fundamental unit of quantity 

 of electricity is about 4.65 XlO"" electro- 

 static units. From these data it is a simple 

 matter to deduce the mass of any atom 

 whose atomic weight is known, and to de- 

 termine the values of a number of related 

 atomic and molecular magnitudes. 



There is now no reason to view the values 

 of these fundamental constants with scepti- 

 cism, but they may be employed with con- 

 fidence in calculations to advance still 

 further our knowledge of the constitution 

 of atoms and molecules. There will no 

 doubt be a great number of investigations 

 in the future to fix the values of these im- 

 portant constants with the greatest possible 



precision; but there is every reason to be- 

 lieve that the values are already known 

 with reasonable certainty, and with a de- 

 gree of accuracy far greater than it was 

 possible to attain a few years ago. The 

 remarkable agreement in the values of 

 e and N, based on so many different 

 theories, of itself affords exceedingly 

 strong evidence of the correctness of the 

 atomic theory of matter, and of electricity, 

 for it is difficult to believe that such con- 

 cordance would show itself if the atoms 

 and their charges had no real existence. 



There has been a tendency in some quar- 

 ters to suppose that the development of 

 physics in recent years has cast doubt on 

 the validity of the atomic theory of matter. 

 This view is quite erroneous, for it will be 

 clear from the evidence already discussed 

 that the recent discoveries have not only 

 greatly strengthened the evidence in sup- 

 port of the theory, but have given an al- 

 most direct and convincing proof of its 

 correctness. The chemical atom as a defi- 

 nite unit in the subdivision of matter is 

 now fixed in an impregnable position in 

 science. Leaving out of account considera- 

 tions of etymology, the atom in chemistry 

 has long been considered to refer only to 

 the smallest unit of matter that enters into 

 ordinary chemical combination. There is 

 no assumption made that the atom itself is 

 indestructible and eternal, or that methods 

 may not ultimately be found for its sub- 

 division into still more elementary units. 

 The advent of the electron has shown that 

 the atom is not the unit of smallest mass of 

 which we have cognizance, while the study 

 of radioactive bodies has shown that the 

 atoms of a few elements of high atomic 

 weight are not permanently stable, but 

 break up spontaneously with the appear- 

 ance of new types of matter. These ad- 

 vances in knowledge do not in any way 

 invalidate the position of the chemical 

 atom, but rather indicate its great impor- 



