B76 TRANSACTIONS OF SECTION A. 
kinetic theory ascribes to the molecules themselves, although even the 
smallest particles examined have a mass which is undoubtedly very large 
compared with that of the molecule. The character of the Brownian 
movement irresistibly impresses the observer with the idea that the par- 
ticles are hurled hither and thither by the action of forces resident in the 
solution, and that these can only arise from the continuous and ceaseless 
movement of the invisible molecules of which the fluid is composed. 
Smoluchowski and Einstein have suggested explanations which are based on 
the kinetic theory, and there is a fair agreement between calculation 
and experiment. Strong additional confirmation of this view has 
been supplied by the very recent experiments of Perrin (1909). He 
obtained an emulsion of gamboge in water which consisted of a great 
number of spherical particles nearly of the same size, which showed the 
characteristic Brownian movement. The particles settled under gravity 
and when equilibrium was set up the distribution of these particles in 
layers at different heights was determined by counting the particles with a 
microscope. The number was found to diminish from the bottom of the 
vessel upwards according to an exponential law—i.e., according to the same 
law as the pressure of the atmosphere diminishes from the surface of the 
earth. In this case, however, on account of the great mass of the particles, 
their distribution was confined to a region only a fraction of a millimetre 
deep. In a particular experiment the number of particles per unit volume 
decreased to half in a distance of 0°038 millimetre, while the corresponding 
distance in our atmosphere is about 6000 metres. From measurements of 
the diameter and weight of each particle, Perrin found that, within the 
limit of experimental error, the law of distribution with height indicated 
that each small particle had the same average kinetic energy of movement 
as the molecules of the solutions in which they were suspended ; in fact, the 
particles in suspension behaved in all respects like molecules of very high 
molecular weight. This is a very important result, for it indicates that 
the law of equipartition of energy among molecules of different masses, 
which is an important deduction from the kinetic theory, holds, at any 
rate very approximately, for a distribution of particles in a medium whose 
masses and dimensions are exceedingly large compared with that of the 
molecules of the medium. Whatever may prove to be the exact explana- 
tion of this phenomenon, there can be little doubt that it results from 
the movement of the molecules of the solution and is thus a striking if 
somewhat indirect proof of the general correctness of the kinetic theory of 
matter. 
From recent work in radioactivity we may take a second illustration 
which is novel and far more direct. It is well known that the a rays of 
radium are deflected by both magnetic and electric fields. It may be 
concluded from this evidence that the radiation is corpuscular in character, 
consisting of a stream of positively charged particles projected from the 
radium at a very high velocity. From the measuréments of the deflection 
of the rays in passing through magnetic and electric fields the ratio e/m 
of the charge carried by the particle to its mass has been determined, and 
the magnitude of this quantity indicates that the particle is of atomic 
dimensions. 
Rutherford and Geiger have recently developed a direct method of 
showing that this radiation is, as the other evidence indicated, discon- 
tinuous, and that it is possible to detect by a special electric method the 
passage of a single a particle into a suitable detecting vessel. The entrance 
of an qa particle through a small opening was marked by a sudden move- 
ment of the needle of the electrometer which was used as a measuring 
instrument. In this way, by counting the number of separate impulses 
communicated to the electrometer needle, it was possible to determine by 
direct counting the number of a particles expelled per second from one gram 
