PRESIDENTIAL ADDRESS. Oo 
of combination in definite proportions. This movement was assisted by the 
possibility of explaining many chemical facts on the basis of thermo- 
dynamics without the aid of any hypothesis as to the particular structure 
of matter. Everyone recognises the great importance of such general 
methods of explanation, but the trouble is that few can think, or at any 
rate think correctly, in terms of thermo-dynamics. The negation of the 
atomic theory has not, and does not, help us to make new discoveries. The 
great advantage of the atomic theory is that it provides, so to speak, a 
tangible and concrete idea of matter which serves at once for the explanation 
of a multitude of facts and is of enormous aid as a working hypothesis. For 
the great majority of scientists it is not sufficient to group together a number 
of facts on general abstract principles. What is wanted is a concrete idea, 
however crude it may be, of the mechanism of the phenomena. This may 
be a weakness of the scientific mind, but it is one that deserves our sympa- 
thetic consideration. It represents an attitude of mind that appeals, I 
think, very strongly to the Anglo-Saxon temperament. It has no doubt 
as its basis the underlying idea that- the facts of nature are ultimately 
explicable on general dynamical principles, and that there must conse- 
quently be some type of mechanism capable of accounting for the observed 
facts. 
It has been generally considered that a decisive proof of the atomic 
structure of matter was in the nature of things impossible, and that the 
atomic theory must of necessity remain an hypothesis unverifiable by 
direct methods. Recent investigations have, however, disclosed such new 
and powerful methods of attack that we may well ask the question whether 
we do not now possess more decisive evidence of its truth. 
Since molecules are invisible, it might appear, for example, an 
impossible hope that an experiment could be devised to show that the mole- 
cules of a fluid are in that state of continuous agitation which the kinetic 
theory leads us to suppose. In this connection I should like to draw your 
attention for a short time to a most striking phenomenon known as the 
‘ Brownian movement,’ which has been closely studied in recent years. 
Quite apart from its probable explanation the phenomenon is of unusual 
interest. In 1827 the English botanist Brown observed by means of a 
microscope that minute particles like spores of plants introduced into a 
fluid were always in a state of continuous irregular agitation, dancing to 
and fro in all directions at considerable speeds. For a long time this effect, 
known as the Brownian movement, was ascribed to inequalities in the 
temperature of the solution. This was disproved by a number of subse- 
quent investigations, and especially by those of Gouy, who showed that the 
movement was spontaneous and continuous and was exhibited by very small 
particles of whatever kind when immersed in a fluid medium. The velocity 
of agitation increased with decrease of diameter of the particles and 
increased with temperature, and was dependent on the viscosity of the 
surrounding fluid. With the advent of the ultra-microscope it has been 
possible to follow the movements with more certainty and to experiment 
with much smaller particles. Exner and Zsigmondy have determined the 
mean velocity of particles of known diameter in various solutions, while 
Svedherg has devised an ingenious method of determining the mean free 
path and the average velocity of particles of different diameter. The 
experiments of Ehrenhaft in 1907 showed that the Brownian movement 
was not confined to liquids, but was exhibited far more markedly by small 
particles suspended in gases. By passing an arc discharge between silver 
poles he produced a fine dust of silver in the air. When examined by 
means of the ultra-microscope the suspended particles exhibited the charac- 
teristic Brownian movement, with the difference that the mean free path 
for particles of the same size was much greater in gases than in liquids. 
The particles exhibit in general the character of the motion which the 
