116 Journal of the Mitchell Society [March 



of the atom is an invariable or constant quantity. It is to be noted 

 however, that the proof is not absolute, and furthermore that while 

 the sum total weig'ht might have been found to be the same the pos- 

 sibility of change in single atoms was not excluded. 



As explanatory of some of his results Crookes, in his classical in- 

 vestigation of the rare earths, threw out the suggestion that in the 

 course of reactions, and such have taken place throughout untold 

 periods of time, changes might be wrought in certain of the atoms 

 under the stresses to which they had been subjected. These he called 

 ''worn atoms." If this were so, then the determinations, in which vast 

 numbers of atoms are dealt with, could give only the average of all 

 atoms concerned as the weight of one. It is manifest that chemical 

 methods alone could not give a satisfactory answer to such a question 

 as this, but after nearly half a century- the same conclusion has been 

 practically reached through the use of modern methods, largely phy- 

 sical. It would seem that in a number of eases at least our accepted 

 atomic weights represent an average weight only. 



The announcement of the Periodic System by Mendeleeff induced 

 a renewed and increased interest in the atomic weights and their de- 

 termination. This sj'stem showed that when the elements were ar- 

 ranged in an ascending series according to the increase in their atomic 

 weights they fell into certain periods and groupings which brought 

 out remarkable relations as to chemical and physical properties. By 

 means of this system Mendeleeff was enabled to predict the existence 

 of certain elements at that time unknown, and to give a fairly accurate 

 description of them and their properties. These predictions were, for 

 the most part, confirmed soon afterwards and the system took its place 

 in science as a natural system. 



It is important to note, however, that in drawing up his ascending 

 series Mendeleeff relied more upon the properties of the elements than 

 upon the atomic weights. His boldness in pointing out places to be 

 filled by yet undiscovered elements testifies to this, and still more the 

 fact that in certain cases where there was conflict between the phys- 

 ical properties and the atomic weight in deciding the position or an 

 element in the series he unhesitatingly adopted the properties as the 

 deciding factor so that so far as these elements are concerned it is not 

 a strictly ascending series of atomic weights. This transposition was 

 quite allowable at a time when most of the atomic weights had been 

 only inadequately investigated. In the cases where this liberty was 



