22 CONTEMPORARY SCIENCE 



cleus of the hydrogen atom. It is very minute in com- 

 parison with the negative, but much more massive. When 

 two free positive electrons are tied together we have the 

 helium atom. We don't know why these positives cling 

 together. 



My last of the great discoveries of modern physics is 

 one that I will just touch upon. It is the discovery of 

 quantum relations in photo-electricity, in X-rays, and in 

 optical spectra ; but here I am coming to a field which we 

 do not know very much about, which we do not yet under- 

 stand, and my main motive in introducing it is to convince 

 you that the physicist, in spite of all he knows, or thinks 

 he knows, is a fairly modest fellow; because there are 

 some things he knows he doesn't know, and one at present 

 is the nature of radiation. However, we know some things 

 about it that are new. For example, it is an extraor- 

 dinarily interesting fact that when light of the X-ray type, 

 or indeed, light of any frequency falls upon say a lithium 

 or sodium surface, or upon almost any surface, it has the 

 property in some way of taking hold of a negative electron 

 in the atoms of that surface and of hurling that electron 

 out with a speed which can be measured, and which we 

 find to be exactly proportional to the frequency of the 

 light. That is an extraordinary phenomenon, and it is 

 one that we explain on a kind of quantum theory which 

 I will not attempt to go into here, because of the fact that 

 we have not yet worked it out fully ; but at any rate, the 

 quantum constant comes out of the photo-electric effect, 

 as shown in my own work, out of X-ray work as discov- 

 ered by Duane and Hunt at Harvard, and out of spectro- 

 scope work, as shown by Bohr in the beautiful theory of 

 the atom which he has developed within the last three or 

 four years. 



