262 Prof. 11. W. Wood on the Electrical Resonance 



a quartz spectrograph. It was found that they were trans- 

 parent to the whole ultra-violet region, even the last two faint 

 cadmium lines which almost all media cut off, appearing on 

 the plate. If a second band is present it must be located in 

 the infra-red region. 



Before continuing the experiments with these highly 

 coloured silver films, it seemed best to establish beyond any 

 doubt that they were analogous in every respect to the films 

 obtained with the alkali metals. One respect in which they 

 differ is the absence of much effect when they are moistened 

 with ligroin. It will be remembered that the films formed 

 by distillation in exhausted bulbs showed most remarkable 

 colour-changes when the hydrocarbon vapour was caused to 

 condense on the wall by the local application of cold. This 

 difference is not of serious moment ; for it is quite possible 

 that the silver particles are already immersed in some medium 

 (possibly gelatine) which prevents the ligroin from entering. 

 As I have mentioned in my previous paper, a very slight 

 change of colour is produced by the ligroin ; but only close 

 examination makes it cAddent. The red films of silver can be 

 changed to purple and blue by the local application of hej^t. 

 This was also true of the alkali metal films. I mentioned in 

 my first communication that where the sodium deposits were 

 very scant the particles scattered a bright green light, the 

 appearance being somewhat as if the inside of the bulb had 

 been washed over with a solution of fluorescene. This light was 

 of the wave-lengths of the light absorbed where the film was 

 thicker. Only films too thin to show any colour by trans- 

 mitted light exhibited this phenomenon. For convenience I 

 spoke of this light as the "fluorescent light,''^ without in- 

 tending to convey the idea that it has any real connexion with 

 fluorescence. 



It remained only to be seen whether silver deposits could 

 be obtained which showed this peculiarity. A plate was 

 flowed with the silver solution, and then partly rinsed off 

 with distilled water and dried. 



By transmitted light there were traces of colour in patches, 

 though most of the plate appeared colourless. On holding 

 it in a beam of strong sunlight in a dark room there appeared 

 many patches which exhibited a bottle-green diffused light, 

 not quite as bright and pure as that shown by sodium, but in 

 every way analogous to it. 



Inasmuch as all of the peculiarities shown by the films of 

 the alkali metals have been observed to a greater or less 

 degree with the silver films, it seems safe to conclude that 

 the cause of the colour is the same in the two cases. 



