68 SCIENCE PROGRESS 



Solids. — (i) Coloured Glasses. — One of the first applications 

 Siedentopf and Szigmondy made of their apparatus was the 

 examination of specimens of glass which had been coloured by 

 gold. When molten glass to which a little chloride of gold 

 has been added is allowed to cool, glass of various colours may 

 be produced. If the cooling is very rapid a colourless specimen 

 is obtained ; but if the glass is cooled more slowly, and kept at 

 a temperature near its melting-point for some time, coloured 

 varieties are produced. Thus at first a rose-colour is shown ; 

 if the cooling proceeds still more slowly, it may be red (the 

 well-known ruby glass), blue, or even brown. The coloured 

 varieties can also be prepared by heating the colourless one. 



Faraday, who had prepared specimens in this manner, found 

 that they produced scattering in a beam of light ; and he sug- 

 gested that the coloration was due to the presence of very small 

 particles, which were beyond the range of his microscope. 



The German physicists mentioned above showed the truth 

 of Faraday's hypothesis. On examining a piece of ruby glass 

 with the ultramicroscope, a number of bright, circular patches 

 of light are observed, clearly proving the presence of small 

 particles in the glass. The colour of the patches changes from 

 one glass to another : in ruby glass it is green, in blue glass 

 yellow or brown. The size of the particles, which is deter- 

 mined by a method to be described later, differs for differently 

 coloured glasses. In the case of the colourless specimens no 

 particles are revealed by the ultramicroscope, but, on heating, 

 they give rise to coloured varieties which are clearly hetero- 

 geneous. 



The above authors have suggested that there are particles of 

 gold present even in the colourless specimens, but, on account 

 of their small size, the light diffracted by them is of feeble 

 intensity. Now it was mentioned above that this intensity is 

 proportional to the square of the volume of the obstacle, so 

 that if n particles unite to form one, the intensity of the light 

 diffracted by a single particle is increased in the ratio n 2 : i. 

 Siedentopf and Szigmondy are of opinion that, in the colourless 

 glasses produced by rapid cooling, there are germs of gold 

 present; and on heating, something analogous to distillation 

 takes place, giving rise to larger visible particles, such as are 

 found in ruby glass. We have a similar phenomenon in the 

 formation of large drops during condensation of a vapour, 



