4 i:\Ar GURAL ADDRESS. 



our perceptions deal immediately with the objects themselves, not the 

 atoms of which they are composed. Can we ever perceive effects upon 

 radiation due to individual atoms as apart from the effect due to their 

 action in bulk ? Tlie answer is, of course, in the affirmativa We may 

 pass a ray of white light through coloured glass or any substance 

 which shows selective absorption, so that the rays of certain wave 

 lengths are removed, and the rest pass on. The result is the sum of 

 separate actions by the billions of atoms of which the body is com- 

 posed, so that the light which emerges may be considered as repre- 

 sentative of light proceeding from each atom after modification 

 therein. Here, then, is a way by which we may hope to learn some- 

 thing of the individual atom. These absorption effects have indeed 

 been closely studied, and have, as is well known, yielded i-esults of the 

 utmost importance not only to pure scientific research but also to 

 commerce and industry. But, as regards the matter we are especially 

 considering, they serve more to open our eyes to the complexity and 

 richness of the inquiry than to yield us laws of any precision or 

 generality. 



In experiments of this kind we make use of sources of radiation 

 external to the atoms, and permit the atoms to modify the original 

 rays. We can, however, force the atoms to become themselves, the 

 primary sources of radiation ; and, in doing so, we avail ourselves of a 

 much more fruitful means of investigation. We may raise substances 

 to incandescence by placing them in a flame, or subjecting them to 

 the more intense heat of the electric arc or discharge ; or we may turn 

 our instruments to the heavens, where glowing suns foi'm fumac«s 

 which far exceed in temperature anything we can find on earth. The 

 atoms are now addressing themselves to us directly; each kind sends 

 us radiation peculiar to its nature and condition. If we could but 

 read the messages ! But we ai*e overwhelmed by the complexity and 

 infinite variability of the effects which we observe. From a bewilder- 

 ing wealth of results we are able to disentangle a few fundamental 

 truths, just enough to make us impatient of our inability to do more ; 

 the work required to elucidate one law successfully seems at the same 

 time to add to the pile of facts yet unclassified and imexplained. The 

 science of spectrum analysis grows year by year. It has taught us of 

 the natures and motions of the stars, and revealed to us fundamental 

 laws of physics ; it has been a keen weapon of chemical research, and 

 given powerful aid to industrial development. But, as to the constitu- 

 tion of the atom, it tells us too much at once; there is a roar of sound 

 from which we can hardly disentangle separate sentences. Not only 

 are the radiations emitted by each atom of exceeding complexity, but 



