BROWNIAX MOVEMENT 81 



that ultra-microscopic particles arc rendered self-luminous. The 

 conditions under which these small particles can be made apparent 

 by this means are that (1) the light scattered is sufficient in intensity 

 and is suitable in wave-length to affect the retina ; (2) the particles 

 differ materially in refractive index from their dispersion medium ; 

 and (3) the particles are not so crowded as to overlap. 



Particles visible under the ordinary microscope are called 

 microns. Smaller particles are termed sub-microns, if they are 

 rendered apparent by the ultra-microscope ; if not, they are 

 amicrons. The smallest particle of gold observed by Zsigmondy, 

 using bright sunlight illumination, was 1-0 ju,/x in diameter. Bearing 

 in mind the large difference in index of refraction between gold and 

 water, this may be considered as the smallest particle ever observed. 

 The table on p. 80 (from Zsigmondy) shows the limits of size of the 

 various classes of particles (Table XII.). 



Properties of Colloids Depending on the Size of the 

 Dispersed Particles 



(ii.) Kinetic. 



{d) The Brownian Movement. The little dots of light seen under 

 the ultramicroscope are not at rest. They dart about hither and 

 thither in a seemingly inexplicable way. According to the kinetic 

 theory of matter, a fluid is assumed to be made up of molecules 

 in a state of very rapid motion and having a mean free path inter- 

 mediate between that of a solid and that of a gas. The colloidal 

 particles in the liquid are hustled into motion by continuous 

 collision with the rapidly moving molecules, of the liquid. If the 

 particles have a natural period of vibration which is a multiple of 

 that of the water molecules, their amplitude of vibration will be 

 increased {e.g. by suitably timing blows on a pendulum its excursion 

 can be increased to a con?iiderable extent. Each blow need be 

 very slight). 



This motion of the particles, while a very striking feature in 

 the field of vision of the ultramicroscope, is not specifically 

 characteristic of colloidal solutions. Particles sufficiently small to 

 be influenced by the high velocity bombardment of the molecules 

 or ions of the solvent may still be well within the limits of visibility 

 under an ordinary microscope. This movement owes its name to 

 its discoverer Brown, a botanist, who described the peculiar oscilla- 

 tion of pollen grains suspended in water in 1827. This Brownian 

 movement may be seen by means of an ordinary microscope in a 

 suspension of the water-colour gamboge, especially when the 

 diaphragm of the microscope is almost closed. The rate of mo ve- 

 il. 



