of Metal Particles for Li (/Jit- Waves. 261 



solution which runs through collected. As it does not keep 

 very well, it is best to prepare it on the day on which it is to 

 be used. 



A sheet of glass is washed clean, rinsed with fresh water, 

 and the w'et surface rubbed over with some shreads of gelatine. 

 It is then drained for a few seconds and dried on a hot plate. 

 A little of the silver solution is flowed over it, the surplus 

 being drained ofi:'. If too much gelatine has been used, pre- 

 cipitation is apt to take place, the deposit taking the form of 

 floating shreads of a reddish membrane. If no considerable 

 precipitation occurs,, the plate, which should have been quite 

 warm when flowed, is placed once more on the hot plate until 

 dry. The Alms formed in this way are usually deep red in 

 colour, though sometimes patches of deep violet form, with 

 sharply defined edges. As one may make a dozen trials 

 without obtaining the violet patches, 1 endeavoured to find 

 some way of producing them at will ; and finally hit upon 

 the right expedient. When the plate is about half dry, with 

 a steaming film and a few small pools of the hot solution, it is 

 removed from the hot plate, held at an angle, and treated 

 with a few drops of alcohol, which are allowed to run down 

 across the still damp portion of the plate. These portions 

 speedily dry into a most gorgeous mosaic of red, purple, and 

 violet patches, the experiment being especially striking in the 

 lantern, as it occupies but a few seconds, and the colour- 

 display spreads over the plate like the blaze of a sunset. So 

 far as I have been able to find, these brilliantly coloured films 

 w^ere never obtained by Carey Lea, though the solution used 

 is one that he employed in his experiments on allotropic silver. 



Any desired depth of colour can be obtained with these 

 films by merely allowing more or less of the solution to 

 remain on the plate. ] have films of such a deep red that 

 they are almost opaque, a gas-flame being barely visible 

 through them. The light which does get through is regularly 

 transmitted, that is, the films are not turbid media. The 

 spectroscope shows that the absorption-band is wider and less 

 sharply defined than is the case with some of the purple 

 potassium films, which have a rather narrow and very black 

 band in the yellowish green. This can be explained by 

 assuming that there is not a great regularity in the size of 

 the particles, and consequently less sharp selective resonance. 

 Inasmuch as some of the sodium films had a bright apple- 

 green colour, in other words absorbed both the red and blue, 

 it seemed worth while to see whether the silver films exhibited 

 the same phenomenon. Red and violet films were formed on 

 a quartz plate, and the absorption-spectra photographed with 



