388 TRANSACTIONS OF SECTION A. 
is a most striking example of accurate physical measurement carried out on an 
engineering scale. His last great work, on the ‘Submechanics of the Universe,’ 
is so original in its ideas and methods that its value cannot yet be fully appre- 
ciated. While it differs so radically from our preconceived ideas that it fails to 
carry immediate conviction, it undoubtedly represents possibilities of truth which 
subsequent workers in the same field cannot afford to ignore. 
The present year has been one of remarkable activity in the world of 
Mathematical and Physical Science, if we may measure activity by the number 
and importance of scientific gatherings like the present for the interchange of 
ideas and the general advancement of science. The celebration of the 250th 
Anniversary of the Foundation of the Royal Society brought to our shores a 
number of distinguished delegates from all parts of the world, to promote the 
ever-growing fellowship among men of science which is one of the surest 
guarantees of international progress. The Congress of Universities of the 
Empire brought other guests from distant British dominions, and considered, as 
one of the principal points in its programme, the provision of facilities for the 
interchange of students between different universities, which will doubtless prove 
particularly advantageous to the scientific student in the higher branches of 
research. In the special branches of knowledge more particularly associated 
with this Section, the International Congress of Mathematics at Cambridge, while 
it affords to Cambridge men like myself a most gratifying recognition of our 
Alma Mater as one of the leading schools of mathematics in the world, has given 
us the opportunity of meeting here a number of distinguished foreign mathema- 
ticians, whose presence and personality cannot be otherwise than inspiring to our 
proceedings, and will compensate for any deficiency in.our own mathematical 
programme. The Optical Convention held this year in London, by the import- 
ance of the papers contributed for discussion, and by its admirable exhibition 
of British instruments, has revealed the extent of our optical industry and talent, 
and has done much to dispel the impression, fostered by an unfortunate trade 
regulation, that the majority of optical instruments were ‘made elsewhere.’ The 
Radio-Telegraphic Conference, held under the auspices of the British Govern- 
ment, has formulated recommendations for regulating and extending the appli- 
cation of the discoveries of modern physics for saving life and property at sea. 
The work of this Conference will be fittingly supplemented on the scientific side 
by the discussion on wireless telegraphy which has been arranged to take place in 
this Section in conjunction with Section G. 
It would be impossible, even if it were not out of place,-for me to attempt 
to review in detail the important work of these congresses, a full account of 
which will shortly be available in their several reports.of proceedings now in 
course of publication. In the present age of specialisation and rapid publication 
it would be equally impossible to give any connected account in the time at my 
disposal of recent developments in those branches of science which come within 
the range of our Section. The appropriate alternative, adopted by the majority 
of my predecessors in this chair, is to select some theory or idea sufficiently 
fundamental to be of general interest, and to discuss it in the light of recent 
experimental evidence. It may sometimes be advantageous to take stock of our 
fundamental notions in this way, and to endeavour to determine how far they 
rest on direct experiment, and how far they are merely developments of some 
dynamical analogy, which may represent the results of experiment up to a certain 
point, but may lead to erroneous conclusions if pushed too far. With this object 
I propose to consider on the present occasion some of our fundamental ideas with 
regard to the nature of heat, and in particular to suggest that we might with 
advantage import into our modern theory some of the ideas of the old caloric or 
material theory which has for so long a time been forgotten and discredited. In 
so doing I may appear to many of you to be taking a retrograde step, because 
the caloric theory is generally represented as being fundamentally opposed to the 
kinetic theory and to the law of the conservation of energy. I would, therefore, 
remark at the outset that this is not necessarily the case, provided that the theory 
is rightly interpreted and applied in accordance with experiment. Mistakes have 
been made on both theories, but the method commonly adopted of selecting all 
the mistakes made in the application of the caloric theory and contrasting them 
with the correct deductions from the kinetic theory has created an erroneous 
