42 THE PROPERTIES OF SOLS AND GELS 



The instrument known as the ultramicroscope is based on the principle 

 of the Tyndall phenomenon. The limit of the resolving power of ordinary 

 microscope is about O.i u. The ultramicroscope can be used for detecting 

 the presence of particles in the size range o.ooi /i, to o.i [x, i.e. in the colloidal 

 range. It is not possible to observe colloidal particles directly in the ultra- 

 microscope; only the light diffracted from their surfaces can be seen. Neither 

 can any definite image of particles in this small range of sizes be obtained. 

 The ultramicroscope is a microscope which is so arranged that the colloidal 

 system or other material to be examined can be illuminated laterally (i.e. at 

 right angles to the tube of the microscope). This lateral illumination is 

 usually provided by a powerful source of light and a suitable series of con- 

 densing and focussing lenses, so arranged that the light is focussed to a point 

 within the mount. Under the ultramicroscope the dispersed particles of a 

 hydrophobic sol appear as bright spots of light varying in size and brilliancy. 

 Very little concerning the actual size or shape of the micelles can be deter- 

 mined since each bright spot represents merely the light diffracted by a single 

 particle. It is possible, however, to determine the number of particles in a 

 given volume of solution by means of the ultramicroscope. The use of the 

 ultramicroscope consists essentially in an observation of the Tyndall phenome- 

 non in a small volume of a sol under the microscope. 



Brownian Movement. — In 1828, the botanist Robert Brown observed 

 through a microscope that pollen grains which were suspended in water 

 showed a rapid oscillatory motion. Brown at first was inclined to attribute 

 this motion to the fact that the pollen grains were alive, but examination 

 of preparations of dead pollen grains and spores showed that they likewise 

 exhibited such a motion. It became evident therefore that this movement was 

 in no way connected with living processes. We now know that any particle 

 up to about 4 or 5 ja in diameter will exhibit this movement when suspended 

 in a liquid. This phenomenon is termed Broivnian movetnent, after its dis- 

 coverer. 



IVIany suspensions in which the particles are within the range of micro- 

 scopic visibility exhibit Brownian movement. It is clearly shown by many 

 of the smaller species of bacteria when suspended in water. In solid-in-gas 

 colloids, such as tobacco smoke, the dispersed particles show a very vigorous 

 Brownian movement. Particles in the protoplasm of slime molds and certain 

 other species frequently exhibit a Brownian movement which is clearly dis- 

 cernible under the microscope. For particles of a given mass, the smaller 

 their volume the greater the amplitude of their Brownian movement. For 

 particles of equal volume, the less their mass the more vigorously they will 

 exhibit Brownian movement. In general this phenomenon is exhibited more 



