C. GENERAL PHYSICS. 127 



white-hot ball cools it becomes first possible to charge it with 

 negative, and subsequently, as it grows cooler, with positive 

 electricity. 1 A ^ February^ 1872. 



. SENSIBILITY OF SELENIUM TO LIGHT. 



The electrician of the Telegraph Construction and Main- 

 tenance Company calls attention to the extreme sensibility 

 of selenium to the influence of light during the passage of 

 the electric current through it. According to this statement, 

 if an electric current be passed through a bar of selenium in 

 the dark, and the bar then subjected to the light of the sun, 

 or even that of a candle, its conductive power is immediately 

 doubled, the eff*ect ceasing, however, when the light is with- 

 drawn. This result is not all diminished by the intervention 

 of rock-salt or colored glass, and is not due in any degree to 

 heat. It is suggested that this discovery is capable of prac- 

 tical application in connection with photometric measurement. 

 18 A, February 21, 1873, 551. 



ANALYSIS OF A FLASH OF LIGHTNING. 



The duration and complex character of flashes of lightning 

 form the subject of a very interesting and valuable contribu- 

 tion by Professor Rood, of Columbia College. To a certain 

 extent his conclusions had been anticipated in a too little 

 known work of Professor Henry on electricity, and also by 

 other observers ; but Professor Rood has, in other respects, 

 penetrated further into the secrets of this phenomenon. By 

 means of a rapidly revolving disk, Rood has shown that " the 

 nature of the lightning discharge is more complicated than 

 has generally been supposed. It is usually, if not always, 

 multiple in character, and the duration of the isolated con- 

 stituents varies very much, ranging from intervals of time 

 shorter than one thousandth of a second up to others at 

 least as great as one twentieth of a second ; and, what is sin- 

 gular, a variety of this kind may sometimes be found in the 

 components of a single flash." The sparks IVom an ordinary 

 electric machine or Leyden-jav are shown by Rood to be 

 much shorter and far more nearly instantaneous, and he failed 

 in several attempts to artificially reproduce the longer dis- 

 charges of the lightning flash by passing sparks through wa- 

 tery vapor or spray. According to the analysis by Dr. Vo- 



