SYMPOSIUM ON COLLOIDS 
45 
One very interesting and important property of colloidal 
solutions can be observed only with the ultramicroscope, an 
instrument which renders the separate particles visible. Under 
this instrument the particles are observed to be in motion, the 
more rapid the smaller they are. This phenomenon is known 
after its discoverer as the Brownian movement. A study of 
this motion has led in the last few years to a virtual proof of 
the molecular theory of matter. A review of the work in this 
field as carried out by Perran and others will not, of course, 
be possible here. Briefly, however, the basis of it is as fol- 
lows : In order to account for the observed prOperties of gases 
the kinetic theory has postulated that at a given temperature 
the kinetic energy of all gas molecules is the same. Since the 
kinetic energy is the product of the mass of the molecule and 
the square of its velocity, the speed with which a large mole- 
cule moves must be smaller than that of a small molecule. On 
the assumption that the colloidal particle is a very large mole- 
cule, it is found that the directly observed velocity of the par- 
ticles has exactly the value predicted by the kinetic theory of 
gases. 
The optical principle of the ultramicroscope, which has 
made this and many other investigations on colloids possible, 
is shown in figure 1. The cell E, which holds the colloidal 
min 
Figure 1. Principle of the Ultra Microscope 
