JANUARY 22, 1914] 
NATURE 
587 
solve this difficulty by denying that the principles | 
of mechanics are true in their application to | 
systems of atomic dimensions. Such a solution 
may appear heroic rather than practical to those 
who have not followed the trend of modern 
physics; those who have know that it is com- 
pletely in accordance with the recent development 
of our ideas. The new conceptions which were 
first introduced by Planck’s theory of radiation, 
and have been applied with such striking results 
to the theory of specific heats and elasticity, are 
directly contradictory of those of the older 
mechanics. They involve the recognition of a 
new “universal constant ” (usually denoted by the 
symbol h), which, like the charge and the mass 
of the electron, is characteristic of all forms of 
matter. The source of many of the difficulties 
connected with the theory of a “positive nucleus ” 
is that such a theory does not introduce sufficient 
quantities to determine an atomic structure; it 
introduces only the charge and mass of an elec- 
tron, and from such quantities neither a length 
(such as the distance apart of the electrons) or 
an energy can be deduced. Thomson’s theory 
rejects the “positive nucleus” and _ introduces 
another quantity, the radius of the atom, but there 
is no reason to believe that it is a ‘universal 
constant.” The newer theories accept the “ posi- 
tive nucleus” and introduce the “universal con- 
stant’ h in place of the radius of the atom. 
Of these theor:es, that of Bohr is the most 
definite. This is not the place to describe the 
precise assumptions made by this theory; it is 
sufficient to say that they are simple, plausible, 
and easily amenable to mathematical treatment ; 
from them all the properties of any atomic system 
which does not contain more than one electron 
can be deduced uniquely.2, There are probably 
only two such atomic systems experimentally 
realisable, the neutral hydrogen atom and _ the 
helium atom, bearing a single positive charge. 
Bohr has calculated the spectra of these systems 
and obtained results which are in exact quantita- 
tive agreement with observation; in respect of 
other properties, the agreement between calcula- 
tion and experiment is as close as can be expected 
in view of the doubts connected with the exact 
connection between these properties and a distribu- 
tion of electrons. The properties of more complex 
atoms cannot he calculated with certainty, owing 
to the mathematical difficulties involved. Indeed, 
theories of atomic structure will probably never 
be very interesting to chemists, for our powers of 
explaining in detail the properties of systems so 
complex as the heavier atoms must be are closely 
limited by the powers of mathematical analysis. 
Bohr’s theory explains more than any previous 
or rival theory, but it does not explain everything. 
It introduces many novel assumptions, of which 
some are quite dubitable, and may have to be 
abandoned. Its great interest lies rather in the 
2 One of the assumptions ori. inally proposed by Van den Broek is especially 
interesting. It is that the number of electrons in an atom in its uncharged 
state is equal to that representing its position in the series of elements 
arranged. in order tof their atomic weights. Thus hydrogen has r electron, 
helium 2, lithium 3, and soon This simple assumption leads to the result 
that the num! er. of electrons is abou half the atomic weight, and, of course, 
t gives a simple reason for that rela‘ion. 
NO. 2308, VOL. 92] 
nature of the ideas which it introduces than in the 
exact explanation of atomic properties to which it 
leads. It not only rejects the principles of 
mechanics, which the most conservative are being 
slowly driven to abandon, but it indicates that 
fundamental propositions are to take their 
place. To attempt to explain Bohr’s theory 
in terms of those principles is useless; it 
is impossible to explain why certain  pro- 
positions are not true by assuming that they are 
true. There are only two alternatives open to the 
modern theoretical physicist: he may either sup- 
pose that the principles of the older mechanics 
are true, and that all the brilliant results which 
have followed from the application of the con- 
ceptions of Planck and Einstein to the most 
diverse phenomena are illusory and devoid of 
evidential value; or he may suppose that they 
are not true. Bohr’s theory offers him the choice 
in its most striking form, Norman CamMpveE.. 
THE AUSTRALIAN MEETING OF. THE 
BRITISH ASSOCIATION. 
g iiiene eighty-fourth meeting of the British Asso- 
ciation will be opened in Adelaide on August 
8, 1914, under the presidency of Prof. W. Bateson, 
F.R.S. On four previous occasions the associa- 
tion has met outside the British Isles; three times 
in Canada, and once in South Africa. Now, for 
the first time, a visit is to be made to the most 
distant portion of the Empire. 
The invitation was conveyed at the Sheffield 
meeting in 1910 by the Australian High Com- 
missioner and Prof. Orme Masson, F.R.S., acting 
on behalf of the Commonwealth Government. 
Since then arrangements have been proceeding for 
the fitting reception in the various Australian 
States of a considerable body of visitors from 
Britain. The sum of 15,000]. has been set aside 
by the Federal Parliament to defray the ocean 
passages of at least 150 members; in addition, 
the Government has undertaken the issue of a 
large handbook of permanent scientific value 
which will contain contributions by Mr. G. H. 
Knibbs, C.M.G., Hon. T. Pearce, M.P., Profs. 
Baldwin Spencer, F.R.S., Edgeworth David, 
F.R.S., Harrison Moore, and many others. The 
State Governments are giving active support in 
granting railway facilities, issuing handbooks 
supplementary to the larger Federal work, and 
in making direct contributions to the local ex- 
penses of the meeting; whilst, of course, every 
university is most heartily adding its full assist- 
ance. 
Official meetings will be held in Adelaide (for 
four days), Melbourne (seven days), Sydney 
(seven days), and Brisbane (four days), extending 
from August 8 until September 1, but the ordinary 
proceedings of sections will take place in Sydney 
and Melbourne only, three sessions being’ held in 
each city. Western Australia is not included in 
the itinerary of the main body of visiting mem- 
bers, but special arrangements are being made 
for an advance party of seventy to visit that State 
between July 28 and August 4. This party will be 
