1873.] On the Electrical Resistance of Selenium. 283 



the production of the line-spectrum of nitrogen, and the expansion of 

 the F line of hydrogen, depend entirely on the intensity of the charge 

 communicated to the Leyden jar. When the pressure of the gas between 

 the electrodes is high, the discharge does not take place until the jar is 

 fully charged ; but as the exhaustion proceeds a less and less charge is 

 communicated to the jar, and the discharge at last is virtually not more 

 than that of the simple current. 



The same effect may be produced by interposing a break in the circuity 

 the length of which may be increased as the pressure in the tube is re- 

 duced. Pliicker and Hittorf appear to have used a break, as in their 

 paper in the Philosophical Transactions, Nov. 1864, they speak of the ex- 

 pansion of lines obtained by increasing the charge of the jar by an inter- 

 posed stratum of air. They do not, however, appear to have noticed that 

 the reduction of pressure in the tube was only equivalent to a diminution 

 of the charge of the jar, and that to this cause many of the changes of 

 spectra which accompany the reduced pressure ought to be ascribed. 



We are continuing our experiments on the effect of temperature on . 

 the spectrum, but prefer to reserve this portion of the subject for the 

 present. 



May 8, 1873. 



FRANCIS SIBSON, M.D., Vice-President, in the Chair. 



The following communications were read : — 



L " The Action of Light on the Electrical Resistance of Sele- 

 nium." By Lieut. Sale, R.E. Communicated by J. N. 

 Lockyer, F.R.S. Received March 28, 1873. 



It having been recently brought to notice that selenium in the crys- 

 talline condition exhibits the remarkable property of having a conduc- 

 tivity varying with the degree of light to which it is exposed, the fol- 

 lowing experiments were undertaken with a view to the further elucida- 

 tion of the matter : — 



Experiment 1. — A bar of crystalline selenium measuring approximately 

 1*5" x *5" X *05" was procured, and platinum wire terminals were fastened 

 to the ends. 



The bar itself was then enclosed in a box having a draw-lid, so as to 

 admit or exclude the light at pleasure. 



Then, the lid of the box being on, the resistance of the selenium was 

 measured by means of a high-resistance galvanometer and a Wheat- 

 stone's bridge, with dial-coils capable of measuring up to 10,000,000 ohms. 

 The battery-power was 2 cells Daniell. 



The measurement was made on a dull cloudy day, and in a room of 

 equable temperature. 



The resistance having been carefully balanced, the lid of the box was 

 withdrawn, when the resistance of the selenium fell instantaneously and 



