ON COLOURS IN METAL GLASSES, ETC. 253 



This view enables us to explain the change of colour from red to blue, by the 

 coagulation of the small spheres, just as in the case of the glasses coloured by gold ; 

 the simultaneous development of a brown reflection and a turbid appearance is at 

 the same time explained. The following quotations must suffice to describe the 

 phenomena in question. 



FABADAY observes that 



"A gradual change goes on amongst the particles diffused through these fluids, especially in the cases 

 where the gold is apparently abundant. It appears to consist of an aggregation. Fluids at first clear, or 

 almost clear, to ordinary observation, become turbid; being left to stand for a few days, a deposit falls."* 



When common salt, or any other substance which dissociates in water, was added 

 to the fluid 



"... The salt diffused gradually through the whole, first turning the gold it came in contact with 

 blue, and then causing its precipitation.! 



" Such results would seem to show that this blue gold is aggregated gold, is., gold in larger particles 

 than before."} 



Again 



" The supernatant fluid in specimens that had stood long and deposited was always ruby . . . there 

 was every reason to believe that the gold was there in separate particles, and that such specimens afforded 

 cases of extreme division." 



Observations made by subsequent physicists agree with those of FARADAY. Thus 

 ZSIGMONDY writes 



" In every case the bright red colour [of suspensions of gold in water] changed to blue on the addition 

 of salt; and decoloration of the upper part of the liquid showed that precipitation has then begun."]] 



Again, STOEKL and VANINO, who examined a large number of suspensions of gold 

 in water prepared by many different methods, state that 



" When the particles [of gold] are very small ... the fluid appears red-yellow, ruby-red. When, 

 however, the particles increase in size, the red and yellow rays are quite cut off and the transmitted light 

 consists only of blue and violet rays, the fluid appearing blue-violet. "U 



* ' Researches in Physics and Chemistry,' p. 414. 



t We may suppose that by friction against the water the gold spheres obtain that negative change 

 which ZSIGMONDY (' LIEB. Ann.,' vol. 301, p. 36) found that they possess. The mutual repulsion of these 

 like charges prevents the spheres from coagulating and thus keeps the gold in suspension in the water. But 

 when an electrolyte is introduced into the fluid, the positive ions discharge the gold spheres, so that 

 coagulation and precipitation result. 



\ FARADAY, loc.. cit., p. 420. 



FARADAY, loc. cit., p. 418. 



|| ZSIGMONDY, ' LIEB. Ann.,' vol. 301 (1898), p. 34. 



II ' Zeitschr. f. Phys. Chem.,' XXX. (1899), p. 108. 



As already stated, STOEKL and VANINO measured the absorption of light, for six different values of \ 

 by a suspension containing a known volume proportion of gold. Using their value of /x(- 000003) to 

 determine the scale of the continuous curve in fig. 1, and comparing the values of wV/A so obtained with 



