260 Prof*. R. W. Wood on the Electrical Resonance 



are prepared and their permanency in air. On attempting 

 to repeat the experiment, I was somewhat annoyed to 

 find that the silver solution dried on the plates with no 

 trace o£ colour, a thin deposit of a black powder being 

 the only result. This powder, if rubbed with the finger, 

 gave a blood-red streak on the plate, which resembled in 

 every respect the coloured deposits obtained last year. I 

 tried cleaning the glass more carefully, using fresh che- 

 micals and distilled water ; but all to no purpose. It finally 

 occurred to me, however, that I had formed the original de- 

 posits on some old lantern-slide plates, from which the gelatine 

 had been removed with hot water. Possible a film of gelatine 

 of infinitesimal thickness might have been the necessary factor. 

 To test this I wetted a sheet of glass and rubbed it over 

 with a small thread of Nelson's photographic gelatine. The 

 plate being cold, only the slightest trace of gelatine could 

 have dissolved in the film of water. The plate was then 

 drained off and dried on a hot metal plate. On flowing this 

 plate with the silver solution and drying it rapidly as before, 

 I obtained a most wonderful film, as red as the densest ruby- 

 glass over most of its extent, but with several patches of deep 

 blue-violet, as rich in colour as dense cobalt-glass. The pre- 

 sence of the gelatine seems in some way to keep the small 

 particles from collecting into aggregates (the black powder). 

 It will be remembered that in my first paper I alluded to 

 an experiment in which the cooling of a bulb, and the con- 

 densation of the hydrocarbon vapour on the coloured film, 

 sometimes destroyed the colour permanently, the minute 

 sodium particles collecting into larger aggregates, which 

 diffracted the light, and showed the pearly lustre, but 

 exhibited no trace of colour by transmitted light. 



As the formation of these films makes a most brilliant 

 lantern experiment, I will give more explicit directions for 

 their preparation. The solution is one described by Carey Lea. 

 Three solutions are prepared ; a 30-per-cent. one of ferrous 

 sulphate, a 40-per-cent. one of sodium citrate, and a 10-per- 

 cent, one of silver nitrate. Fourteen c.cms. of the citrate 

 solution are mixed with 10 c.cms. of the ferrous sulphate 

 solution, to which is then added 10 c.cms. of the silver 

 nitrate solution. A dense black precipitate at once forms, 

 and the whole is at once poured into a filter. As soon 

 as the liquid has entirely run through, the precipitate is 

 washed with 10 cms. [not more) of distilled water. This is to 

 remove the salts which make the precipitate insoluble. After 

 the water has entirely passed through the filter, about 25 c.cms. 

 of distilled water are poured into the filter, and the blood-red 



