PHYSICS MILLIKAN. 183 



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 understand, 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 extraordinarily 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 one of the atoms of that surface and of hurling that electron out 

 with a perfectly definite speed, a speed which we can measure 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 enter into 

 here because of the fact that we have not yet worked it out fully, so 

 that I can not give you anything very definite about it ; 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 shown by Duane and by 

 D. L. Webster at Harvard, and out of spectroscope work, as shown 

 by Bohr in the beautiful theory of the atom which he has developed 

 within the last three or four years. 



I think I have brought you in this brief survey to the very out- 

 most boundaries of our present knowledge. Bring me back 10 years 

 from now, and we will know more about these quantum theories, but 

 for the present I will stop, and close this hasty survey of the prob- 

 lems and successes of modern physics with a few reflections which 

 are based upon historical studies. 



At the University of Chicago I have a friend by the name of 

 Dr. Breasted, who is an Egyptologist. Now Dr. Breasted tells me 

 that he and his fellow Egj^ptologists have proof that less than 100 

 years elapsed from the time when, about 5,000 years ago, the Egyp- 

 tians knew so little about building that the best they could do was to 

 pile crude rows of uncut stone around their dead, to the time when 

 some of the great pyramids were built, structures which represent in 

 some ways the height of the builder's art, structures on which the 

 surfacing is so perfect that huge granite blocks 18 feet on a side are 

 joined together without cement, and with not as much as one one- 

 hundredth of an inch of space anywhere between them. That kind 

 of engineering we do not do now, luckily we do not have to do it, but 

 it is doubtful if we could do it if we would. I am mentioning this to 

 bring out the fact that Egypt, at that time, got the key to a certain 

 kind of development, and pushed that development to a marvelous 



