Travers. — Presidential Address. 121 



is often created by the indiscriminate use of the terms "atom" 

 and " molecule." The word "atom " is properly applicable only 

 to matter in its ultimate condition of divisibility, and the word 

 "molecule " to a particle of matter formed by the chemical com- 

 bination of two or more atoms. The number of possible com- 

 binations is practically infinite, and I have long thought that 

 the lines in the spectrum indicate the more or less complex 

 nature of such combintitions, in the case of each of the 

 substances experimented upon, those which give the fewest 

 lines being less complex in molecular structure than those 

 which present many lines. Professor J. J. Thomson, in a 

 paper published in the Philosophical Magazine in October, 

 1893, points out that the electrification of a gas is not a mere 

 mechanical process. In his view it is a chemical or quasi- 

 chemical one, which goes on within its molecules. He con- 

 ceives that part at least of the materials must be split up into 

 atoms before electricity can be carried, for he shows that new 

 combinations are formed when the dissociated atoms part 

 with electricity. He maintains that a molecule of a gas, qud 

 molecule, cannot be electrified. 



Two important questions at once arose from these demon- 

 strations, — namely, What is an electrified particle ? What 

 progress is being made in physical research when we find that 

 electrified particles are substituted, by electrical excitation, 

 for the body of which they had been the component parts? 

 These questions have not yet been wholly solved, but Crookes's 

 experiments show clearly enough that the medium for the 

 transmission of electricity must consist of ponderable matter 

 in some form or other. Lord Kelvin, in commenting on these 

 experiments and demonstrations, remarks that they had led 

 him to the conclusion that the molecule contains both what 

 are called ether and ponderable matter; though, as already 

 observed, I think it much more probable that the supposed 

 ether is neither more nor less than the ultimate form of 

 matter, and that the molecule of any of the so-called simple 

 bodies of chemistry is composed of atoms of some one homo- 

 geneous substance in a special condition of combination. 



It was whilst engaged in similar researches, following the 

 lines of discovery propounded by Hertz, that Eontgen noticed 

 the peculiar nature and some of the properties of the dark 

 radiations to which he gave the name of X rays. Now, when 

 we speak of these rays as "dark radiations," we are using 

 the term "dark" merely with reference to the limited visual 

 capacity of the human eye, which, as is well known, can'only 

 perceive light- waves that are not shorter than eir^oo °^ ^^ 

 inch and not longer than twice that length, and it is by 

 reason of this limitation in the capacity of the human eye 

 that the ultra-violet rays at the violet end of the spectrum 



