COLLOID CHExMISTRY. - 205 



Other metallic and iuoi'ganic Eols appear to contain particles of the 

 same order of magnitude as the gold sols, but different sols vary con- 

 siderably, and any sol contains a mixture of particles of various sizes. 



As seen by the ultra-microscope, the particles are not at rest, but in 

 constant rapid vibrating movement, and, according to Zsigmondy, gold 

 particles have also a translatory motion. This vibration, which certainly 

 plays its part in the physics of colloid sols, is identical with the 

 Brownian or pedetic motion common to all minute particles suspended in 

 liquid, but more marked in the ultra-microscopic, since it increases 

 inversely with the size of the particles. This motion increases with 

 temperature and diminishes with increased viscosity of the liquid, but is 

 permanent and unaffected by time, or apparently by the exclusion of 

 radiant heat and light. All theories therefore which ascribe the energy 

 involved to outside iniluences would seem to be excluded, but it mu^t be 

 remarked that it is impossible to observe the phenomenon without light, 

 and, ultra-microscopically, without light of a very intense character, so 

 that the view suggested by Regnauld ' that it was due to the warming of 

 the particles by radiant heat cannot perhaps even yet be entirely ignored. 

 Sir W. Ramsay - suggested that the motion was directly due to the heat- 

 vibration of the water-molecules, though he thought it necessary to 

 assume highly complex water-molecules to account for the disturbance of 

 the much heavier solid particles. Einstein ^ and Smoluchowski ** have 

 discussed the phenomenon mathematically from two entirely different 

 physical standpoints, and have reached results which not only show a 

 scarcely expected concordance with each other, but a close relation to 

 observed facts. The experimental work of Svedberg •'^ also strongly 

 supports this view, so that, without presuming to discuss the mathe- 

 matical evidence, it appears that a strong case is made out for regarding 

 the motiim as a direct consequence and evidence of the kinetic theory of 

 heat. It may be remarked that if external sources of energy can be 

 excluded, heat-motion seems the only available cause, since all the energy 

 involved in overcoming internal friction of the liquid is converted into 

 diffused heat, whieh in a space of uniform temperoture cannot be 

 ret-ansformed into any othe- form of energy. 



If an electric current of sufficient strength be passed through an 

 aqueous electrolysable solution in which the anode and cathode are 

 separated by a diaphragm of porous earthenware, the liquid will pass 

 through the latter towards the cathode till a certain equilibriun? 

 pressure is established. The phenomenon is known as 'electrical osmose ' 

 or 'cataphoresis.' Although observed by Eeuss in 1809, the subject was 

 first svstematically investigated by Wiedemann.*' He showed that, with 

 uniform difference of pressure, the quantity of liquid which passed 

 through the diaphragm was proportional to the current-strength and in- 

 dependent of the area or thickness of the porous plate ; and the pressure 

 attained, when equilibrium was reached, to the potential ditlerence 

 between the two sides of the diaphragm. Quincke,^ substitutnig a 



• J. d. Pharr.1. (3), 18oT, 34. 141. ■ Chom. Xcvs, 1892, 65, 00. 



^ Ann. F/i!/s. (4). 1905, 17, 549-560 ; and 1906, 19, 2. 



' JMd. (4), 1906, 17, 756-780. 



" Ark. fir Kemt, Min. och Gcol, 1907, 2, Kr 29 and 34. 



« Ann. Plnjs. (2), 1852, 87, 321; and Die LeJire der MeMrizitnf, IbO.., LJ. I,, 

 993-1019. ' - ;^ 



' ' .\Hn. Phjs. (2), 18G1, 113, 513 .V.)S, 



