48 REPORTS ON THE STATE OF SCIENCE.—1917. 
compounds. It is probable that their formation is incomplete, and this 
would explain their not obeying the law of multiple proportions. 
Other workers besides Krafft and Biltz are of the opinion that many 
dyestuffs form colloidal solutions. Teague,5° Buxton, and Vignon base 
their conclusions with Krafft on diffusion experiments, Pelet-Jolivet 5 
and Wild on their ultramicroscopic studies. Knecht and Batey and 
later Donnan and Harris ** deny the colloidal nature of benzopurpurine, 
chrysophenine, and Congo red on grounds that their electrical conduc- 
tivities are normal and that they exert osmotic pressure. The work of 
Donnan and Harris is important and leads to the view that measurements 
of the osmotic pressure are of no value unless account be taken of the 
presence of an electrolyte (if present). The electrolyte distributes itself 
unequally on the two sides of the dialysing membrane and sets up an 
opposing osmotic pressure. Zsigmondy, considering their experiments, 
also those of Bayliss,®4 Teague and Buxton,®> Rachlmann v. Vegesack,°® 
Biltz and Bredig, places Congo red and all the other azo- dyestuffs of its 
class among the colloids, owing to their low diffusibility and optical visi- 
bility (in the ultramicroscope). On boiling Congo red dissociates; it 
is extremely sensitive to the presence of carbon dioxide in diffusing, but 
in the matter of conductivity and osmotic stress it behaves like a crystal- 
loid or electrolyte ; the conclusion being that the salt is dissolved as 
. molecules, but the presence of extremely minute amounts of electrolytes 
causes aggregation into submicrons. Dyestufis with the alizarin group 
are highly colloidal, while the dyestuffs containing sulphonic groups not 
only form very soluble compounds in water, but their salts are very 
dialysable even with molecules of from 76-78 atoms, yet, according to 
Biltz,5? molecules of from 70 to 95 atoms should not diffuse. 
In the light of these facts, all the recent research into the properties 
of colloids has its direct bearing upon the theory of dyeing. 
Following upon Krafit, P. D. Zacharias published in 1900 a * Farben- 
theorie,’ of which the German translation appeared in 1908. His theory 
meanwhile developed in a series of essays in the ‘ Farbenzeitung,’ 1901, 
12, 149 and 165, and ‘ Ber. d.d. chem. Ges.’ 1905. His earlier theory is 
one of colloid interdiffusion plus adhesion. He discounts the chemical 
theory altogether, objects to the solid solution theory pure and simple, 
but, recognising the colloidal nature of the dyes and of the fibres, he 
suggests an interdiffusion plus precipitation, followed by or resulting in 
an adhesion, whose nature he does not particularise. His later theory, 
which he insists does not essentially vary from his first statements, follows 
upon the researches of Graham, Krafft, Justin-Mueller, Biltz and Zsig- 
mondy (Biltz and Zacharias variously quarrelled about the originality 
of each other’s theory and experiments; while Zacharias continuously 
and pugnaciously defends his theory, it is Biltz who brought forward 
the experimental material for such a theory—no such experimental data 
5° Zeitschr. fiir Phys. Chem. 1907, 60, p. 419. 
51 Zeitschr. Chem. Ind. Koll. 1908, 8, 174. 
52 Journ. Soc. Dyers and Col. 1909, 25, 194. 
53 Chem. Soc. Trans. 1911, 99, 1554. 
54 Proc. Roy. Soc. 1909, 81B, 269. 
55 Toc. cit. 
56 Zeitschr. physikal. Chem. 1910, 78, 481. 
57 Tbid., 1911, 77, 91; also Gedankboek, van Bemmelen, 1910, 108. 
