54 REPORTS ON THE STATE OF SCIENCE.—1917, 
acid changes the shade to chocolate-brown, without, however, causing 
any precipitate. *Alexander has examined these colour-changes at the 
ordinary temperature in the ultra-microscope, and reports—dilute benzo- 
purpurin solution shows a field full of ultra-microns which from their 
brilliancy and motion appear to be 50-60yp in size. When a little 
acid is added, the ultra-microns gradually gather together into clumps, 
or groups, whose motion decreases as their size increases, until the whole 
of the dye is deposited in coagulated masses of bright ultra-microns. 
Stronger acid causes instant coagulation in large masses when acid is 
added to the gelatine dye solution; no change is produced unless the 
acid is strong enough to cause a more or less extensive agglutination 
of ultra-microns into small groups of 2 or 3, which, however, have still 
sufficient motion to keep afloat. The cause of the variation in the colour- 
changes produced by immersion in dilute acid on the different fibres dyed 
with benzopurpurin is due (according to author) to their difference in 
protecting action on the adsorbed dye. A practical application for 
the use of protective colloids is found in Feilmann’s English patent, 
10,693, 1906, where the employment of casein is made to produce colloidal 
solutions of various unsulphonated dyestuffs, particularly azo- dyes. 
But this application has not found much use up to the present. 
Similariy Mohlau and Zimmermann? produce colloidal indigo by 
means of protalbumic and lysalbumic acids—also Fabrik v. Heyden 
produce the same substance by the use of various protein substances.1® 
The phenomenon of ‘ contact electrification’ and its value in dyeing 
remains. Whenever two substances solid, liquid, or gaseous are in contact, 
having a common surface, all kinds of forces come into play, which 
are difficult indeed to study when acting between solid and solid or solid 
and liquid or solid and gas, but between liquid and gas have been formulated. 
Surface tension and surface electrification are the problems which parti- 
cularly concern colloidalists, the surfaces between the disperse and the 
continuous phase being so immense in comparison with the mass. 
Gibbs?°* enunciated the theorem ‘Those substances which lower 
the surface tension of a solution with respect to another phase must 
concentrate themselves upon the latter.’ This can be experimentally 
proved, with gas-liquid and liquid-liquid systems, for it is possible to 
measure their surface tensions; but Freundlich has lately shown that 
those substances which lower the surface tension between two liquids 
are strongly adsorbed, not only by these but also by solid Lodies ; hence 
the rule would seem to have general application. Moreover, the adsorp- 
tion-isotherm (showing quantitative adsorption by solid bodies) will 
hold also for the gas-liquid and liquid-liquid boundary. Freundlich, 
Schmidt 1°? and Svante Arrhenius 1° have given formulae for this adsorp- 
tion, which within certain limits holds for the adsorption of crystalloids 
by charcoal and by hydrogels.4°® Ultra-microns of one colloid likewise 
adsorb ultra-microns of another colloid, and even when they both bear 
the same charge; this can be seen in the ultra-microscope and goes 
104 Zeitschr. f. Farben und Teatil Chemie, 1903, 25. 
105 German Patent Application, 112,051, 1903. 
106 Thermo-Dynamische Studien, p. 321. 
107 Zeitschr. f. phys. Chem. 1911, 77, 641-660; 1912, 78, 667-681. 
108 Meddelanden fran K. Vetenskapsakad. Nobelinstitut, \9J1, 2 N. 7. 
109 Van Bemmelen. 
