SECTIONAL TRANSACTIONS.—A. 273 
less ; the misconception which gives rise to it is due to the lines of force 
being endowed with a physical reality for which there is no justification. 
When the bar magnet rotates the electrons within it move relatively to one 
another through magnetised space and thus experience forces, but no 
meaning can be attached to the movement or non-movement of the magnetic 
condition of space which undergoes no change in magnitude or direction. 
The statement recently made by Prof. Cramp, that Faraday’s description of 
an experiment was lacking in detail because ‘ he omitted the possibility of 
the e.m.f. being produced by a rotating magnetic flux cutting the stationary 
parts of the circuit,’ is unfair to Faraday, who could hardly be expected to 
foresee that such a queer misconception would subsequently arise. Prof. 
Cramp admits after making about fifty (!) experiments that they are incon- 
clusive, as indeed they must be since they were designed to answer a 
meaningless question. Lines of force and tubes of magnetic induction are 
mathematical fictions: there is nothing material about them, nor do they 
represent discontinuities in space, which could be earmarked in order to 
detect their movement. Their number is a mere convention. 
Friday, September 7. 
PRESIDENTIAL AppREsS by Prof. H. M. Macponatp, O.B.E., F.R.S., on 
Theories of Light (10.0). (See p. 19.) 
Dr. F. W. Aston, F.R.S.—The roll-call of the isotopes (11.0). 
The word ‘ isotopes’ was first used by Soddy to indicate atoms having 
identical chemical properties but different mass which he discovered among 
the products of radioactivity. Their presence in ordinary stable elements 
was definitely proved later by the mass-spectrograph. Of recent years the 
word has altered its meaning and is now used to designate any atomic 
species. By the study of mass-spectra, supplemented in a few cases by that 
of optical spectra, the analysis of the common elements may now be regarded 
as fairly complete. The main isotopic constituents are known for all but 
four—palladium, iridium, platinum and gold. The accuracy of the data 
varies in a wide degree from element to element, the analysis being easiest 
technically for the inert gases, and most difficult for the rare earths and noble 
elements. Disregarding those of radioactive period less than one million 
years, the total number of isotopes now known is well over 240, about three 
per element. The isotopic complexity of elements of odd atomic number 
shows a remarkable regularity. Excepting hydrogen, none of these has 
more than two isotopes. On the other hand, elements of even atomic 
number may be much more complex, tin having as many as eleven isotopes, 
and it is an interesting speculation whether or not the number may be 
extended indefinitely by increasing the delicacy of the methods of detection. 
Discussion on The structure of alloys (11.30) :— 
Prof. W. L. Brace, O.B.E., F.R.S.—Introduction. 
We may conveniently define an alloy by two characteristics The first 
is the arrangement of the positions occupied by its metal atoms. A different 
geometrical pattern of the atomic sites characterises each phase of the alloy 
system, and is the essential feature which remains constant in a single-phase 
region although the composition of the phase may vary over a wide range. 
The second is the distribution of the atoms of each kind in a binary or 
L2 
