422 



NATURE 



[December 17, 1914 



blue. Moreover, the ingredients used can, on one 

 hand, be varied considerably without affecting- the 

 colour, and, on the other, batches of exactly the 

 same composition may vary in colour from the 

 desired blue to a g-reenish-white owing- to varia- 

 tions in the process of fusion. The suggestion 

 has been made, and supported by a g-ood deal of 

 evidence, that these variations may be due simply 

 to differences in the "degree of dispersity,'* i.e., 

 in the size of the particles of one constituent dis- 

 seminated throug-h another. The same explana- 

 tion has been tentatively put forward by Wo. 

 Ostwald to account for the peculiarities of a very 

 interesting group of dyes, the sulphide dyes — 

 peculiarities which are not easily accounted for 

 by their constitution alone. 



It is, of course, well known that certain in- 

 organic pigments, obtained by precipitation, vary 

 in colour according to the concentrations and 

 temperatures of the reacting solutions : thus, 

 cadmium sulphide may be any shade from salmon 

 pink to golden yellow. The reason is again to 

 be found in the different size of the particles of 

 precipitate which — up to certain limits — tends to 

 increase with decreasing concentrations. There 

 is a possibility of still further reducing this size 

 with resulting changes in tint and incidentally 

 w*ith an increase in covering power, by adding 

 to the solutions extremely small amounts of some 

 organic colloids like glue or casein — a procedure 

 which deserves extensive investigation by those 

 interested. 



Among the most important and most exhaus- 

 tively investigated properties of colloids are their 

 electrical characteristics. To describe these as 

 briefly as possible, we may say that finely divided 

 substances in contact with water and a few other 

 liquids become electrically charged. In the 

 majority of cases the charge is negative, but the 

 oxides and hydroxides of a number of metals and 

 a few dyestuffs are positive. The amount and 

 eventually the sign of these charges can be modi- 

 fied by the addition of acid or alkali : thus, nega- 

 tively charged particles become more so in alkaline 

 media, while acid in increasing amounts diminishes 

 and finally neutralises negative charges. Both 

 these effects find technical application : thus, very 

 fine precipitates or slimes which obstinately refuse 

 to settle in neutral or alkaline liquids come down 

 rapidly if the latter are made acid. The effect of 

 acidity is to neutralise the negative charges on 

 the particles, and their mutual electric repulsion, 

 which helps largely to keep them in suspension. 

 On the other hand, the addition of alkali to clay 

 slip (the particles of which are also negative) has 

 the opposite effect of suspending or "dispersing" 

 the particles, so that the slip loses its plasticity 

 and can be cast in moulds, a procedure which is 

 employed on a large scale. 



Owing to the charge on particles in contact 

 with a liquid — which, as need scarcely be said, 

 assumes the opposite charge — a relative movement 

 of the two phases takes place in an electric field, 

 that is, when electrodes connected to a source of 



NO. 2355, VOL. 94] 



current are immersed in the liquid. If the par- 

 ticles are freely suspended, they xnove towards the 

 electrode having the opposite sign ; if, as is 

 sometimes the case, the solid is aggregated into 

 a porous diaphragm, the liquid flows through the 

 diaphragm towards the other pole. To take a 

 concrete example : particles of carbon are nega- 

 tive and therefore travel towards the positive pole 

 or anode ; if they are formed into a fixed dia- 

 phragm, for instance, a plate of porous carbon, 

 the water — which is positively charged — flows 

 through this diaphragm to the negative pole or 

 kathode. The movement of particles in the 

 electric field is called cataphoresis ; the flow of 

 liquid through a diaphragm electro-osmosis. Both 

 phenomena have been applied industrially by 

 Count Schwerin. Thus, clays can be freed from 

 iron oxide by being suspended in water in an 

 electric field, since clay particles are negative 

 while iron oxide is positive, so that the two sub- 

 stances travel to opposite electrodes. Spongy 

 substances havmg a cellular structure, like peat^ 

 can be freed from water by being placed between 

 a perforated plate acting as kathode while a 

 second plate resting on the peat serves as anode. 

 The solid being negative, the water is positive 

 and flows to the kathode, escaping through the 

 perforations in the latter. Both processes are 

 capable of varied and extensive applications, many 

 of which are being developed. 



It is highly probable that electric factors play 

 a considerable, although by no means clear, part 

 in the various processes for separating sulphide 

 ores from gangue with the aid of oil, particularly 

 in the Minerals Separation process, which consists 

 in adding to a slightly acid pulp made from the 

 ore, part of which must be exceedingly fine, a 

 minute quantity of oil and agitating in such a 

 manner that a considerable quantity of air be- 

 comes mixed with the liquid. As soon as agita- 

 tion ceases the air rises in the form of bubbles 

 which carry the sulphide to the top, while the 

 gangue sinks to the bottom. The necessity for 

 acid reaction, in conjunction with known differ- 

 ences in the electrical properties of oil, quartz and 

 air surfaces in contact with acid water, is strong 

 evidence that the electrical properties of suspended 

 particles play an essential part in the process, and 

 its future investigation will have to take them into 

 account to a much greater extent than has been 

 the case up to the present. 



Our knowledge of the effects of the two factors 

 discussed so far — size and electric state of par- 

 ticles — has been gained through the study of 

 colloidal solutions, chiefly of metals and sulphides, 

 i.e., of suspensions in which the diameter of the 

 particles lies belov/ the limit of microscopic visi- 

 bility, roughly speaking, below 200 /x/j. (i /XyU = one- 

 millionth millimetre). While the preparation of 

 such solutions is of the greatest theoretical 

 interest, and some are used therapeutically, they 

 haA-e found only a few technical applications 

 worth mentioning. 



E. Hatschek. 



