p 



THE PROPERTIES OF COLLOIDS 143 



chemical reactions such as those studied in the simpler compounds usually 

 dealt with by the chemist. 



OPTICAL" BEHAVIOUR OF HYDROSOLS. Nearly all colloidal solu- 

 tions present what is known as the Faraday- Tyndall phenomenon. When 

 a beam of light is passed through an optically homogeneous fluid, the course 

 of the beam is invisible. A beam of sunlight falling into a dark room is 

 rendered visible by impinging on and illuminating the dust particles in its 

 course. Each of these particles, being illuminated, acts as a centre of dis- 

 persion of the light, so that the course of the beam is apparent to a person 

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

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

 was polarised. This 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 position the light is bright, in the position at right angles to this 

 it becomes dim or is extinguished. The production of the Tyndall pheno- 

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

 scopic particles, varying in size from 5 to 50 jnu. 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 raffindse (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 /C or -3 JLL, 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. 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 Brown- 

 ian movements. The smallest particles which can be seen show a combined 

 movement, consisting of a translatory movement, in which the particle 

 passes rapidly across the field in one-sixth to one-eighth of a second, and a 

 r$ovement 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 the two 

 hases. 



