276 POPULAR SCIENCE MONTHLY. 



that the distinction between colloidal solutions and suspensions is not 

 one of quality, but only one of degree. 



Properties related to the Electrification of the Particles. — A quite 

 distinct class of properties may be next considered, which depend not 

 on the size of the colloidal* particles, but apparently upon the presence 

 of electric charges upon them. 



The most direct evidence of this electrification is furnished by 

 the migration of the colloidal particles through the liquid under 

 the influence of an applied electromotive force. This effect may 

 be well illustrated with colloidal suspensions of arsenious sulphide 

 and ferric hydroxide contained in two U-tubes. The tops of the 

 U-tubes are covered with goldbeaters' skin and are surrounded by 

 wider tubes containing pure water in which platinum electrodes 

 are placed, so that the products of electrolysis collecting around them 

 may not influence the colloid. These tubes are then connected in 

 parallel with the terminals of a 110-volt circuit in such a way that 

 the current will flow through each of them in the direction from 

 left to right. It is some minutes before any result is observed. 

 Then it is seen that the ferric hydroxide has moved down with a sharp 

 surface of demarkation on the side where the current enters, leaving a 

 clear layer of water above, and that the arsenious sulphide has done the 

 same, but on the opposite side of the tube. In other words, the ferric 

 hydroxide particles are moving with the positive current towards the 

 cathode, the arsenious sulphide with the negative current towards the 

 anode. The former are, therefore, positively, and the latter negatively, 

 charged. These results are typical ones : such movement, or migra- 

 tion, as it is commonly called, is exhibited by all colloidal suspensions, 

 and, it may be added, also by fine microscopic suspensions, like those 

 of quartz, kaolin and lampblack. Other basic hydroxides, like those 

 of aluminum, chromium and thorium, and certain dyestuffs, migrate to 

 the cathode just as does the hydroxide of iron. The suspended par- 

 ticles of almost all other substances, whether colloidal or microscopic, 

 migrate to the anode. This is true, for example, of silicic acid, stannic 

 acid, metallic sulphides, salts like silver iodide and Prussian blue, and 

 metals like gold and platinum. Of special interest with reference to 

 the explanation of the phenomenon is the recently discovered fact that 

 an egg-albumen suspension migrates towards the cathode in an acid 

 liquid and towards the anode in an alkaline one. 



In regard to the cause and character of the electrification two as- 

 sumptions deserve consideration : one is that it is simply an example 

 of contact electricity, the colloid particle assuming a charge of one 

 sign and the surrounding water that of the other. This correlates this 

 phenomenon of migration with that of electrical endosmose; for the 

 motion of suspended kaolin, for example, through water against the 



