THE PROPERTIES OF COLLOIDS 161 



can be easily tested by looking at the beam through a Nicol's prism. 

 If the prism be slowly rotated, it will be found that, while at one posi- 

 tion the light is bright, in the position at right angles to this it becomes 

 dim or is extinguished. The production of the Tyndall phenomenon 

 may therefore be regarded as a test for the presence of ultra-microscopic 

 particles, varying in size from 5 to 50 IJL/JL. The phenomenon is 

 perhaps too sensitive to be taken as a proof that a fluid presenting 

 it is a suspension rather than a solution. It is shown, for instance, by 

 solutions of many bodies of high molecular weight, such as rafnnose 

 (a tri-saccharide) or the alkaloid brucine (Bayliss). 



A particle having a diameter less than half the wave-length of 

 light, i.e. about 300 \ or -3 /m, cannot be clearly distinguished under any 

 power of the microscope. The fact that an ultra-microscopic particle 

 may serve as a centre for dispsrsal of light may be used for rendering 

 such particles visible under the microscope. For this purpose a strong 

 beam of light is passed in the plane of the stage of the microscope 

 through a cell containing the hydrosol, which is then examined under 

 a high power. The arrangement for this purpose was first devised 

 by Zsigmondy and Siedentopf . On examining with this apparatus a 

 dilute gold sol, we see a swarm of dancing points of light, " like gnats 

 in the sunlight," which move rapidly in all directions, rendering it 

 almost impossible to count their number in the field. The coarser 

 particles present slight oscillations similar to those long known as 

 the Brownian movements. The smallest particles which can be 

 seen show a combined movement, consisting of a translatory move- 

 ment, in which the particle passes rapidly across the field in one- 

 sixth to one-eighth of a second, and a movement of oscillation of 

 much shorter period. The representation of the course of such a 

 particle is given in Fig. 28. 



The size of the smallest particles seen in this way may amount to 

 005 JUL. Not all colloidal solutions show these particles in the ultra- 

 microscope. In some cases this is due simply to the small size of the 

 particles, and the addition of any substance, which causes aggregation 

 and therefore increase in the size of the particles, will bring them into 

 view. In others the absence of optical inhomogeneity may be due 

 to the coincidence of the refractive indices of the two phases of the 

 hydrosol, or to the absence of any surface tension and therefore dividing 

 surfaces between the two phases. 



ELECTRICAL PROPERTIES OF COLLOIDS 



In the case of many hydrosols the ultra-microscopic particles 

 of which they are composed carry an electric charge which, according 

 to the nature of the solution, may be either positive or negative. 

 On this account, the particles move if placed in an electric field, and 



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