December 31, 1914] 



NATURE 



483 



properties do not depend simply on the concen- 

 tration, but on the whole previous treatment, 

 especially as regards temperature, and they are 

 not in any event stable, but vary with the age 

 of the solution, although no chemical change 

 whatever may be demonstrable. It is therefore 

 of the utmost importance that a solution, such as 

 the cellulose or nitrocellulose solutions used in 

 making artificial silk, should possess identical 

 properties at a given moment, and research has 

 shown that the most sensitive and most easily 

 measured " indicator " constant is the viscosity. 

 Viscosity measurements accordingly form a recog- 

 nised means of control in these and other indus- 

 tries. The theoretical study of viscosity is also 

 gradually throwing light on the mechanism of the 

 changes which colloidal solutions undergo in 

 ageing. 



A very general property of finely divided matter, 

 and therefore of colloids par excellence, is the 

 power of taking dissolved substances out of solu- 

 tion. This phenomenon, so far as it consists in 

 a concentration of such dissolved substance on 

 the surface of the colloid, is called adsorption, 

 and is of the greatest importance in nature and 

 in the arts. The use of Fuller's earth and of 

 gelatin or isinglass for decolouring and clarifying 

 liquids is one of the familiar applications : other 

 instances are so numerous that the difficulty is 

 one of selection. Thus the processes of dyeing 

 and of tanning begin with adsorption, which may 

 be followed by chemical reaction or by diffusion 

 of the substance, concentrated in the first instance 

 on the surface, into the body of the fibre. Accord- 

 ing to the personal bias of investigators each of 

 the factors involved has been credited with pre- 

 ponderating importance, and it will require much 

 patient work to settle their relative share in pro- 

 cesses so complicated. Adsorption, as distinct 

 from chemical combination, has also been ad- 

 vanced as the explanation of certain peculiarities 

 in the vulcanisation of rubber, and forms a subject 

 of rather acute controversy. The adsorptive 

 action of various constituents of the soil on the 

 salts necessary to plant life, a line of investigation 

 first pursued by van Bemmelen, is gradually 

 becoming clear, and light is thereby thrown on 

 a subject of extraordinary intricacy. The so-called 

 humus substances, in particular, have received 

 attention recently, and there seems to be sound 

 evidence in favour of the view that their effect 

 is largely due to*their adsorbent action rather 

 than to their somewhat indefinite acid character. 

 In photographic chemistry the proof that the 

 photohaloids, i.e. the coloured products obtained 

 by exposing certain forms of silver halogen salts 

 to light, are adsorption compounds of silver and 

 silver salt, has been established experimentally by 

 preparing these products from silver salt and 

 colloidal silver. Incidentally, it may be remarked 

 that the connection between degree of dispersity, 

 mentioned in the last article, and colour has pro- 

 vided the explanation for the great range of shades 

 of the silver image which can be obtained by 



NO. 2357, VOL. 94] 



varying the exposure and the concentration of the 

 developer. 



We have so far considered colloids chiefly in 

 a state of suspension or solution, and have now 

 to refer briefly to the equally important " gel " 

 condition, i.e. the more or less solid form they 

 assume when solutions of sufficient concentration 

 are allowed to set by cooling, e.g. gelatin and 

 agar, or when the solvent is removed by some 

 means. The most familiar example is the gelatin 

 jelly. These gels possess a number of projjerties 

 of great technical importance. Many of them 

 retain their cohesion and elastic properties when 

 the greatest part of the solvent is removed, so 

 as to yield elastic filaments or films ; cellulose 

 and several of its derivatives are thus used in the 

 manufacture of artificial silk and of photographic 

 films. The elastic properties can be varied con- 

 siderably by the addition of other soluble sub- 

 stances to the solvent : one of the most important 

 instances is celluloid, consisting of nitrocellulose 

 and camphor or various related bodies. Another 

 important feature of certain gels is that of being 

 rendered insoluble and also incapable of absorbing 

 water by various agents : thus casein treated with 

 formaldehyde becomes a very stable plasmic mass, 

 used industrially as "galalith," while gelatin, to 

 which a small amount of a bichromate has been 

 added, becomes insoluble on exposure to light — 

 a property utilised in the "carbon" process of 

 photographic printing and other methods of photo- 

 graphic reproduction. 



Since gels contain very large percentages of 

 liquid the diffusion of dissolved substances in them 

 is very little impeded, as was known already to 

 Graham. Solutions of two reacting substances 

 may, therefore, easily be brought into contact in 

 a gel, and reactions carried out in these conditions 

 exhibit a number of very striking and important 

 peculiarities, of which space permits us to mention 

 only one. By a suitable choice of concentrations 

 the reaction may be slowed and may be arrested 

 at an intermediate stage, which in aqueous solu- 

 tions would be quite transitory. Perhaps the 

 most striking instance is the extremely sensitive 

 silver bromide of the photographic dry plate, 

 which cannot be produced at all in aqueous solu- 

 tion. It is very probable that this effect of gels 

 may become of technical importance in many other 

 directions, and it deserves careful consideration. 



Although the sur\'ey here given has necessarily 

 been very brief and incomplete, it is probably 

 adequate to show in how many directions industry 

 is likely to profit by research in colloidal chemis- 

 try. To sum up briefly, the latter has demon- 

 strated the bearing on chemical processes and 

 their results of a number of factors which are in 

 no sense of the word "chemical," such as the 

 degree of dispersity; the electrical properties of 

 finely divided matter and the possibility of modify- 

 ing them by the reaction of the medium ; the 

 effect of the latter on such universally important 

 phenomena as swelling of organic substances and 

 on their elastic properties and, finallv, the in- 



