182 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



aluminum with known elements in the order of their weights — we 

 can not investigate the lighter ones by the X-ray method because their 

 frequencies are too low ; at least we have not yet found how to investi- 

 gate them. Now, there are just 12 lighter elements than aluminum, 

 so we may put them all in, starting with hydrogen. That would 

 make hydrogen correspond to the top step. The second way is to 

 find arithmetically the greatest common divisor of the square root 

 frequencies. This gives us a frequency which is within a few per 

 cent of the highest frequency which hydrogen can produce, according 

 to Lyman's measurements of optical radiations in the ultraviolet 

 region. This indicates again that hydrogen is the element corre- 

 sponding to the first step. All of this seems to mean three things, 

 it means first that the X rays of hydrogen are just its ordinary visible 

 radiations; second it means that Moseley opened up a whole new 

 field of radiation, beginning with the radiations of hydrogen, and ex- 

 tending up to a frequency (92) 2 or 8161 times as high as that given by 

 hydrogen. I have squared 92 because 92 is the number of the step 

 corresponding to uranium, the heaviest known element, and the one 

 having the highest frequency in its characteristic X rays. 



Moseley's discovery means in the third place, almost certainly, that 

 the elements are built up one from another by successively adding 

 the nucleus of the hydrogen atom. The probable reason for the change 

 in frequency as the nucleus takes on a stronger and stronger charge 

 is that the electron sending off say the highest characteristic fre- 

 quency is in a stronger electrical field in the helium atom, for ex- 

 ample, than in the hydrogen atom, and so, as the charge on the nu- 

 cleus goes up by successive steps in going from element to element, 

 the characteristic frequencies go up by corresponding steps. 



We may then picture with considerable confidence this whole 

 physical world as built up out of one positive and one negative 

 electron. The positive electron is the nucleus of the h} T drogen 

 atom. It is very minute in comparison with the negative, but much 

 more massive. When two free positive electrons are tied together 

 we have the nucleus of the helium atom. We don't know why these 

 positives cling together. We can assume, as an hypothesis, that there 

 are four positives in helium which are held together by two negatives, 

 thus leaving but two free positives as experiment indicates is the case. 

 The assumption here is that in the nucleus one negative is capable 

 of holding two positives. This assumption would make the nucleus 

 of any atom contain a number of negatives equal to the atomic num- 

 ber and a number of positives equal to twice the atomic number. So 

 much for a very brief and incomplete sketch of Moseley's contribu- 

 tion to modern physics. 



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 



