Metal Particles for Light- Waves. 429 



than before. This was found to be the case. The con- 

 ductivity of the films is surprisingly good. A strip of a deep 

 blue film measuring 10 by 5 mms. had a resistance of only a 

 little over 2 ohms. After heating the film its resistance 

 dropped to about half this value. 



It is even possible to change the colours by heating with- 

 out sending the film over to the gold-leaf stage. I have a 

 yellow film which local heating has changed to blue and 

 purple in spots. A similar temperature change was noticed 

 with the sodium and potassium films, as I mentioned in my 

 previous paper, which I was unable to explain satisfactorily, 

 since it appeared to be the opposite to what we should expect, 

 if it was due to evaporation, which was the only thing that 

 occurred to me at the time. It now seems as if the changes 

 in both instances could be referred to a partial coalescing of 

 the resonators. 



I find a difficulty in reconciling the assumed resonance of 

 the films with their electrical conductivity, but the matter is 

 perhaps less troublesome than the explanation of why gold- 

 leaf transmits any light at all. 



Coloured films of a similar nature can be made of silver, 

 by pouring a solution of Carey Lea's soluble allotropic silver 

 on a glass plate and evaporating it over a flame*. Ten parts 

 of the ferrous sulphate solution are mixed with fourteen parts 

 of the sodic citrate solution, to which is then added ten parts 

 of the silver nitrate. The precipitate is allowed to settle, 

 the liquid decanted, and the residue filtered. A small amount 

 of distilled water is next poured into the filter and allowed 

 to run through. More distilled water is now passed through 

 the filter and collected. It will be found to have a very deep 

 red colour, and if a small amount be spread over a clean 

 glass plate and evaporated, it leaves a film which appears deep 

 red, purple, and blue by transmitted light. The reflected 

 light is of a complementary tint, the purple film reflecting 

 green, and the red a greenish blue. 



Ligroin changes the optical properties of the film, though 

 the change is only to be seen when a nicol is used, and even 

 then it is not very pronounced. I feel pretty certain that I 

 have detected traces of a granular structure in these silver 

 films with the microscope, and hope to confirm the observa- 

 tion as soon as a more perfect objective is at my disposal. 



An investigation of the dispersion of the films, which I 

 intend to take up next, and a more careful study with polarized 

 light, will doubtless throw further light on the matter. 



Baltimore, Md., U.S.A., 

 May -'50, 1902. 



* American Journal of Science, vol. xxxvii. p. 476 (1889). 



