150 ANNUAL RECOKD OF SCIENCE AND INDUSTRY. 



ON ELECTRIC CONDENSATION. 



Neyreneuf has investigated the subject of condensation of 

 electricity by the use of the so-called condensers of various 

 kinds. He concludes: 1. That the constancy of the charge 

 of an electrophorous ordinarily depends upon the imperfec- 

 tion of contact. 2. That the employment of a proof plane is 

 thoroughly defective in any quantitative research, and even 

 in any qualitative research into the electric state of an iso- 

 lated lamina. 3. That the employment of the gold-leaf elec- 

 troscope demands the greatest precautions, because of the 

 variable condition that is always produced in consequence 

 of the action as an electrophorous of the isolating lamina. 

 4. That an electrophorous used under the most favorable con- 

 ditions theoretically, Avill not give any result, because of the 

 antagonism of its spontaneous discharge, and that obtained 

 by the ordinary working of the api^aratus. 6 B, 1873, 202. 



THE EFFECT OF LIGHT ON THE PASSAGE OF AN ELECTRIC 



CURRENT. 



Mr. Willoughby Smith, a well-known electrician of En- 

 gland, has made some observations on the remarkable prop- 

 erty of the metal selenium, which have been repeated by oth- 

 ers with, however, variable success. Mr. Smith states that, 

 beino; desirous of obtainino- a convenient source of electric 

 resistance, he was induced to experiment with bars of sele- 

 nium, whose diameter was about the twenty-fifth part of an 

 inch, and length half an inch ; each bar was sealed in a glass 

 tube, and its ends connected with platinum wire. Noticing 

 great discrepancies in his results, he found on investigation 

 that the resistance of these small bars altered materially ac- 

 cording to the intensity of the light that so shined upon 

 them. When the bars were fixed in a box with a sliding 

 cover, so as to exclude all light, their resistance Avas at its 

 highest, and remained so constantly ; but immediately upon 

 removing the cover of the box, the resistance diminished 

 from 15 to 100 per cent., according to the intensity of the 

 light falling upon the bar. Merely intercepting the light from 

 an ordinary gas-burnei-, several feet from the bar, increased 

 the resistance from 15 to 20 per cent. If the light was in- 

 tercepted by rock-salt, or clear glass, the resistance varied 



