228 Mr. G. T. Beilby. The Effects of [June 4, 



covering the whole surface is distinctly seen. Under the microscope 

 this film was of a very pale pink colour, the thicker parts being of a 

 slightly deeper shade. The opaque-looking granules of the photograph 

 were really of a dark brown-green colour. 



In figs. 16 and 15 the thickened patches are of larger size and 

 of greater opacity. The transparent parts seemed as if covered 

 with an irregular film of pink jelly dotted over with little rounded 



The bright gold and platinum paints made for ceramic gilding by 

 the Gold und Silber Scheide Anstalt of Frankfurt, supply an excellent 

 means of obtaining continuous films of almost any thickness. In 

 these paints the metal has been brought into perfect solution in an 

 essential oil. A plate of glass or of mica is smoothly coated with the 

 paint, which is allowed to dry naturally. It is then heated in an air 

 bath to about 400, when the solvent oil and other volatile constituents 

 are driven off and a smooth bright metallic film is found firmly adhering 

 to the glass. At this stage the film is translucent and green by trans- 

 mitted light. If the film is kept at a higher temperature for some 

 time it becomes more transparent, and the colour by transmitted light 

 changes to blue or purple. By long continued heating the film assumes 

 a frosted appearance by reflected light, and the colour becomes paler 

 by transmitted light. 



These films are sometimes blistered by air bubbles, and an excellent 

 idea of , their continuity and transparence is obtained by carefully 

 examining the spot where a bubble has been formed. So horn-like 

 and translucent are these films that it was at first assumed that the 

 paint contained a binding flux or frit, but, on careful inquiry I was 

 assured by Dr. Fritz Koessler, the scientific director of the Frankfurt 

 Works, that the films left on the glass are pure gold and platinum and 

 free from any flux or frit. Fig. 14 is a gold film of this kind which 

 has been annealed to the purple stage. 



The thickness of a similar film was estimated by weighing the gold 

 from a given area. Assuming that the density of the gold in this 

 form is normal, the thickness was 166 ft/x, or about twice the thickness 

 of a gold leaf. Under the microscope the thickness, as seen at bubbles 

 and broken edges, seemed to be much greater than this. 



The phenomena of annealing are equally well seen in silver leaf and 

 in films of silver chemically deposited on glass. In the case of silver, 

 annealing takes place at the comparatively low temperature of 300 to 

 350. 



When the films are very thin the granular aggregations are not 

 visible by transmitted light. As the thickness increases, the aggrega- 

 tions increase in distinctness, and are seen to be brown by transmitted 

 light. The very feeble metallic reflection of these brown granules 

 is noteworthy. 



