THE PROPERTIES OF COLLOIDS 145 



standing on one side of it. Tyndall showed that, if the particles wei 

 sufficiently minute, the light dispersed by them at right angles to the bean 

 was polarised. This can be easily tested by looking at the beam through 

 a Nicols prism. If the prism be slowly rotated, it will be found that 

 while at one position the light is bright, in the position at right angles to t h,s 

 ; becomes dim or is extinguished. The production of the Tyndall ph-no- 

 menon may therefore be regarded as a test for the presence of ultra-mi< ..,- 

 scopic particles, varying in size from 5 to 50 w. 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 raffinose (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 ^, cannot be clearly distinguished under any power of 

 the microscope. The fact that an ultra -microscopic particle may serve 

 as a centre for dispersal 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 //. 

 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 th- two 

 phases. 



ELECTRICAL PROPERTIES OF COLLOI-DS 



In the case of many hydrosols the ultra-microscopic particles of which 

 they are composed carry an electric charge which, according to ti 

 of the solution, may be either positive or negative. On this 

 particles move if placed in an electric field, and the direction of t 

 ment reveals the nature of their change. Thus colloidal feme hvdn 



