234 
DR. A. E. H. TUTTON ON THE 
all the rubidium and ammonium salts dealt with by the author, and it is most 
satisfactory to have it confirmed in such an incontrovertible and authoritative 
manner. Incidentally also, the value of molecular volume and of topic axial ratios, 
when used, as the author has always solely done, for the strictly comparable 
members of isomorphous series such as those now under discussion, is confirmed 
and enhanced. 
The bearing of the very interesting work of Prof. Ogg and Mr. Hopwood on the 
theory of valency volumes is dealt with in a separate memoir.* 
[Added, February 7, 1917 .] 
With equal validity “ atomic number” can be substituted for “atomic weight ” in 
the statement of the author’s law of progression of the crystallographic properties. 
For the differences between potassium, rubidium, and caesium are similarly related 
as regards both functions, rubidium standing half-way between potassium and 
caesium in both respects, as will be clear from the following comparison :— 
K. 
Rb-K. 
Rb. 
Cs - Rb. 
Cs. 
Cs - K. 
Atomic weights 
38-85 
46 
84'9 
47 
13F9 
93 = 2 x 46‘5 
Atomic numbers 
19 
18 
37 
18 
55 
36 = 2 x 18 
The differences between 
K and 
Rb, and 
Rb 
and Cs, are 
thus equal for 
constants, namely, 46 or 47 in atomic weight and 18 in atomic number, and the 
difference between the two extreme members of the family group, potassium and 
caesium, is double as much, namely, 93 for atomic weight and 36 for atomic number. 
The atomic number, the sequence number of the element, when all the elements 
are arranged in order of ascending atomic weight in the periodic table, has acquired 
great significance from the work of Moseley, who in two memoirs on “ The High 
Frequency Spectra of the Elements ” f lias shown definitely and experimentally that 
the atomic number represents the value of the charge N of positive electricity on 
the atomic nucleus. For we already knew from the work of Sir J. J. Thomson, 
Sir E. Rutherford, Barkla, van den Broek, and Bohr, sufficient concerning the 
structure of the atom to render it certain that there is an inner positively charged 
nucleus, surrounded by a number of negatively charged electrons approximately 
equal to half the atomic weight, and together equivalent electrically to the positive 
charge on the nucleus ; and van den Broek and Bohr had suggested that the 
nuclear charge N would prove to be equal to the atomic number. Now N increases from 
the atom of one element to the atom of the next in the periodic table always by a 
* ‘ Roy. Soc. Proc.,’ A, vol. 93, p. 72 (1917). 
t 1 Phil. Mag.,’ series 6, vol. 26, p. 1024 (1913), and vol. 27, p. 703 (1914) ; see also “ Obituary Notice,’ 
by Sir E. Rutherford, ‘ Roy. Soc. Proc.,’ A, vol. 93, p. xxv. (1917). 
