ULTRAMICROSCOPY 77 



in the liquid, but Gouy clearly showed that this could not 

 be their cause. When every precaution is taken to avoid 

 these currents, the movements are as pronounced as ever. 

 Moreover, the motions of neighbouring particles are usually 

 quite independent. Hence some other cause must be sought. 



It has been mentioned above that the intensity of the move- 

 ments increases as the particles diminish in size, and Gouy 

 further showed that it also increased as the temperature rises. 

 These facts remind us of the movements of the molecules of 

 a gas on the ordinary kinetic theory of gases, which increase 

 with temperature and are more intense, the smaller the mass 

 of the molecule ; and the above author suggested that the 

 Brownian movements are due to collisions between the mole- 

 cules of the liquid and the particles in suspension. 



It is generally accepted that the molecules of a substance are 

 in rapid motion. At intervals collisions occur between the 

 different molecules, and as a result of these collisions their 

 velocities are continually changing. If there are particles in 

 suspension in the gas or liquid, it can be imagined that the 

 molecules in the course of their excursions collide with them ; 

 and as a consequence the particle is set in motion. Now these 

 particles in suspension are many times larger and heavier 1 than 

 the molecules which collide with them, so that, though the 

 molecules may be moving, on the average, with very great 

 velocity (in hydrogen at 'o°C, for example, the velocity is 1-84 

 kilometres per sec), the velocity acquired by the suspended 

 particle may be small. 



Again, although a particle may, in a given short interval 

 of time, be struck by a large number of molecules, it cannot 

 a priori be assumed that there will be any co-ordination between 

 the individual effects, so that the resultant effect may be small. 

 Further, we have seen that the particles make excursions of the 

 order of 10 p, distances which are large compared with the 

 mean free path of the molecules of the gas or liquid. 2 Gouy 

 assumed that there did exist some co-ordination between the 

 collisions, so that a large effect might eventually be produced. 



1 A particle of gamboge in a colloidal solution of that substance has a mass 

 io 9 times as large as that of a molecule of hydrogen. 



2 In hydrogen at normal temperature and pressure, the mean free path is 

 about o'2 fj. ; and the distance between the molecules of water at ordinary 

 temperatures is probably of the order I /*/*. 



