52 REPORTS ON THE STATE OF SCIENCE.—1917. 
property of electrolytes is well known to be due to their dissociation into 
negative anions and positive cations ; the charge on the colloid particles is 
not so easy to explain, as they are not generally supposed to be ionised. 
Yet Zsigmondy gives three various explanations, and Bredig % and 
Billitzer %* assume the same, 7.e., difference of dielectric constant between 
particle and intermicellary fluid ; capture (e.g., colloid gold with H ions) or 
deposition of ions. Hardy ® has the distinction of pointing out that it is 
the electrification of the particles of any irreversible colloid that causes it 
to remain in solution ; if an electrolyte of opposite charge be added till the 
isoelectric point be reached, the colloid will coagulate into a gel ; the ions of 
the electrolyte bearing the opposite charge to the colloid particles also take 
part in the precipitation.°® Schulze %7 has pointed out, and this has been 
many times corroborated, that the valency of the ion is of great influence in 
this coagulating process: a trivalent ion is worth 1,000 monovalent or 30 
divalent ions. The curve is the same as the adsorptions curve, as Freund- 
lich %8 has pointed out, and Pelet-Jolivet’s °® table of comparisons between 
the laws of contact electrification, colloid coagulation, dyeing and capillary 
attraction bears out in a remarkable way. Not only do crystalloids 
precipitate colloids, but two colloids of opposite charge will precipitate each 
other, unless one is in overwhelming excess of the other ; there is a more or 
less wide zone within which mutual coagulation will take place, though 
there is only one point at which the charges actually neutralise each other. 
For dyeing, this rule is of the widest importance .1°° 
If the dry gel of a reversible colloid like gelatine or agar be put into 
water to be dissolved, it behaves in a manner quite distinct from a crystal- 
loid. The latter gives off particles from its surface till all is dissolved, in a 
manner analogous to the evaporation or sublimation in a gaseous medium, 
and the solution obeys Boyle-Gay-Lussac’s laws of gases, within certain 
limits. The gelatine swells up and absorbs the water into its own sub- 
stance, but does not dissolve until the temperature rises to above 25° C. 
Van Bemmelen and Batschli believed that jellies possessed a porous 
structure of microscopic dimensions, but this theory is now abandoned ; 
such pores can be produced by irregular contraction under the action of 
alcohol or chromic acid, and the structure of natural jellies is molecular, and 
they are of the nature of solid solutions. Capillarity may cause imbibition 
but cannot produce swelling in itself, though it may liberate the elasticity of 
the imbibing substance, as water does that of a dried sponge. 
Similar phenomena will explain the adsorption of liquids by charcoal, 
unglazed pottery, &c., though, as von Georgievics points out, not the 
decolorisation of liquids: this is, according to Knecht and Suida,? due to 
the activity of the nitrogen compounds of the charcoal, and to the acid 
nature of the silicates. 
82 Kolloid Chemie, p. 48. 
83 Anorganische Fermente, 1901, p. 16. 
" Zeitschr. f. Hlecktrochemie, 1902, 8, 638-642; Zeitschr. f. phys. Chemie, 1903, 
45, 307-330. 
"9 Loc. cit. 
96 See also Dreaper, Chemistry and Physics of Dyeing, p. 123. 
97 Journ. f. praktische Chemie, 1882 (2), 25, 431-452; 1883, 27, 320-332. 
93 Zeitschr. fiir phys. Chemie, 1910, 73, pp. 385-423. 
” Theorie des Farbeprozesses, 1910. 
100 See Zsigmondy, Koll. Chem., Ch. 98, ‘ Kolloidfallung der Farbstoffe,’ 
0! Sitzungsber. d. Akal, d. Wiss., Wien, 1904, 113, 1lb, 725-761, 
