78 



SCIENCE 



[N. S. Vol. XLI. No. 1046 



some of Bohr's assumptions. So high an 

 authority as Jeans^^ calls it "a most in- 

 genious and suggestive, and I think we 

 must add convincing explanation of the 

 laws of series spectra," and yet he adds a 

 little later that the only justification for 

 the assumptions Bohr makes is "the very 

 weighty one of success." Eutherford 

 cautiously observes: 



The theories of Bohr are of great interest and 

 importance as a first attempt to construct atoms 

 and molecules and explain their spectra. 



The views of Rutherford and Bohr re- 

 garding the structure of atoms are strongly 

 supported by some striking experiments of 

 Moseley published during the past year.^^ 

 His work utilizes the methods worked out 

 by W. H. and W. L. Bragg^" for measuring 

 the spectra obtained by reflecting X-rays 

 from the faces of crystals. Barkla and 

 Sadler-" showed in 1908 that if X-rays 

 from an ordinary tube fall on different 

 metals, "characteristic X-rays" are given 

 off — these being different for each metal. 

 Many metals can give out at least two dif- 

 ferent types of radiation. Barkla called 

 these the "K series" and the "L series" 

 radiations. For each metal the "K" radi- 

 ation is about 300 times as penetrating as 

 the " L " radiation. Kaye^^ has shown that 

 an element excited under suitable condi- 

 tions by rapid cathode rays gives out a con- 

 siderable portion of the X-rays produced 

 in the form of characteristic rays. 



Moseley photographed the spectra ob- 

 tained by using a great variety of different 

 metals as targets for cathode-ray bombard- 

 ment. The X-rays so produced were re- 

 flected from a crystal face and then fell 

 upon the photographic plate. Spectra of 

 the third order showing fine sharp lines 

 were obtained. Similar results were se- 

 cured for over forty metals. For the ele- 

 ments of lower atomic weights, each spec- 

 trum showed two prominent lines, and the 



spectrum of any element was almost ex- 

 actly like that of the element next below it 

 in the periodic table except that it was 

 shifted in the direction of shorter wave 

 length by about the distance between its 

 two lines. The radiation was of the "K" 

 type. Thus a close relation was estab- 

 lished between the X-ray wave-length and 

 chemical properties. Further, the fre- 

 quency of the principal line was found 

 to be proportional to {N-a)^, where N is an 

 integer and a is a constant equal to about 

 unity. N is called the atomic number of 

 the element. Thus is it 20 for Ca, 22 for 

 Ti, 23 for Va, 24 for Cr, 25 for Mn, 26 for 

 Fe, 27 for Co, 28 for Ni, 29 for Cu, 30 for 

 Zn, etc. These numbers are very nearly in 

 the orders of the increasing atomic weights, 

 but more exactly in the order of Men- 

 deleeff's periodic table. The numbers then 

 correspond with the changes in chemical 

 properties more nearly than do the atomic 

 weights. For instance, we have Fe, Co, Ni 

 representing both the chemical order and 

 order of the atomic numbers (26, 27, 28), 

 while Fe, Ni, Co is the order of increasing 

 atomic weights. It thus appears that this 

 atomic number is a more fundamental quan- 

 tity than is the atomic weight, or as Soddy^^ 

 has put it, 



It is the nuclear charge rather than the a,tomie 

 mass, which fixes the position of the element in 

 the Periodic Table. 



A. van der Broek^^ had before this sug- 

 gested that the total number of unit 

 charges on the electrons of an atom is the 

 number representing the position of the 

 element arranged according to increasing 

 atomic weight. But in a neutral atom the 

 sum of the (negative) charges on the elec- 

 trons should equal the positive charge on 

 the nucleus, so that the two statements 

 amount to the same thing. 



When the experimental values found for 

 the frequency were compared with those 



