Electron Atmospheres of Metals, 317 



minute air-gaps at very low voltages, though no data are 

 given as to the precautions taken to exclude the presence of 

 foreign matter between the plates, or how the exact distance 

 between them was determined. 



The experiments which I am about to describe were the 

 outgrowth o£ a piece of work on the electrical properties of 

 very long heat-waves (112 yu, obtained by focal isolation) 

 which has occupied a portion of my time during the past 

 winter. Having found that quartz plates thickly coated with 

 very fine copper dust, or with closely packed droplets of 

 mercury, were perfectly transparent to the heat-waves, 

 opacity commencing only when the size of: the metal par- 

 ticles exceeded one quarter of the wave-length, I commenced 

 experiments with silver films deposited on quartz and ruled 

 with a diamond point into a microscopic checker-board. 

 The rulings were made for me by Dr. J. A. Anderson, who 

 has worked out the technique of ruling thin metal films to 

 such a point that clean cats, quite through the metal, of a 

 width certainly no greater and probably less than the wave- 

 length of ultra-violet light are possible. The great trouble 

 in making very close rulings on thin metal films is the tearing 

 of the metal, which causes the cuts to run together. Suc- 

 cessful rulings occur only when a natural edge of the diamond 

 is used and the edge set accurately parallel to the direction 

 of the cut. The silver was deposited on thin plates of quartz, 

 the thickness being what is usually described as " half- 

 silvered," and the plates were found to be absolutely opaque 

 to the heat-waves. A cross ruling was then made breaking 

 the film up into squares measuring about 0*1 of a wave- 

 length on a side, i. e. 0*011 mm. From the results of the 

 experiments with the mercury droplets I expected to find 

 the waves freely transmitted, but the ruled film was found 

 to be quite as opaque as before. The electrical resistance of 

 the film was then compared with that of an unruled one with 

 a box-bridge and galvanometer, and found to be practically 

 the same. This result seemed most astonishing, for the cuts 

 when examined with a 1/12 inch oil-immersion objective 

 were found to be clear through the film, there being no con- 

 ducting bridges across the gaps. 



A second film was now deposited on a glass plate, and 

 wiped off with the exception of a strip about 3 mm. in width, 

 terminating at each end in a larger patch of silver, against 

 which the electrodes of silver-leaf rolled into soft balls were 

 clamped. About 20 diamond cuts were made across the 

 narrow strip, and its resistance measured. Before the cuts 

 were made the resistance was 10 ohms, which was increased 



