APPARATUS FOR THE ELECTRIC LIGHT. 141 



conductive, even of induced currents, to be capable of 

 passing a current able to produce the electric light. To 

 remedy this defect of conductivity, the plate of kaolin must 

 be warmed in the neighbourhood of the electrodes, and this 

 is done in a very simple manner, by connecting the two iron 

 clips mentioned above by a rod of retort carbon. By first 

 taking a spark from one of the clips the carbon glows and 

 transmits its heat to the neighbouring part of the kaolin, 

 which fuses and affords a passage to the electric influence, 

 at first through a very short space (i or 2 millimetres), then 

 over one progressively longer in proportion as the carbon 

 is moved along the kaolin, and which at length occupies 

 the whole length of the latter when the carbon has reached 

 the second iron clip. The current then follows the track 

 of fused matter which is progressively formed, and reveals 

 itself to the eye as a band of dazzling light, appearing much 

 wider than it really is on account' of irradiation. Care 

 must be taken to concentrate the heat developed by the 

 carbon by means of a reflector of refractory matter, which 

 may consist of a plate of kaolin. The light thus supplied is, 

 as I have already said, very steady, very brilliant, and much 

 softer than the arc light. Its power depends, of course, on 

 the resistance of the circuit and on the number of luminous 

 points interposed, but with a weak electric force it is equal 

 to one or two gas-jets. 



Kaolin appears to be the best substance, because, being 

 prepared as a paste, it can be made very homogeneous; but 

 other substances are capable of producing the same effects; 

 magnesia and lime have indeed given very good results.* 



* The conductivity of this kaolin, studied by means of the process used 

 in my researches on substances of mediocre conductivity, showed traces of 

 the passage of the voltaic current produced by 12 Leclanche elements only 

 when the specimen had remained in a cellar for more than a day. When 

 it was kept in an inhabited apartment it gave no deviation, and when it was 

 heated to redness in a spirit-lamp it gave a deviation of only i degree. It 

 is therefore necessary in order to obtain the important effects that have been 

 mentioned, that the electricity of tension should, in consequence of the '-esist- 



