46 



Professor J. A. Fleming 



[Feb. 14, 



Fig. 14. 



charges. One of tlie facts he brought forward * was that a bright 

 red-hot iron ball, well insulated, could be charged negatively, but 

 could not retain for an instant a positive charge. He showed this 

 fact in a way which it is very easy to repeat as a lecture experiment. 

 Here is a gold-leaf electroscope, to which we will impart a positive 

 charge of electricity, and project the image of its divergent leaves on 

 the screen. A looker, the tip of which has been made brightly red- 

 hot, is placed so that its incandescent end is about an inch from the 

 knob of the electroscope. No discharge takes place. Discharging 

 the electroscope with my finger, I give it a small charge of negative 

 electricity, and replace the poker in the same position. The gold 

 leaves instantly collapse. Bear in mind that the extremity of the 

 poker, when brought in contiguity to the knob of the charged electro- 

 scope, becomes charged by induction with a charge of the opposite 

 sign to that of the charge of the electroscope, and you will at once 

 see that this experiment confirms Prof. Guthrie's statement, for the 

 negatively-charged electroscope induces a positive charge on the 

 incandescent iron, and this charge cannot be retained. If the induced 

 charge on the poker is a negative charge, it is retained, and hence 

 the positively-charged electroscope is not discharged, but the nega- 

 tively-charged electroscope at once loses its 

 charge. Pass in imagination from iron balls to 

 carbon molecules. We may ask whether it is a 

 legitimate assumption to suppose the same fact 

 to hold good for them, and that a hot carbon 

 molecule or small carbon mass just detached 

 from an incandescent surface behaves in the 

 same way and has a greater grip for negative 

 than for positive charge ? If this can possibly 

 be assumed, we can complete our hypothesis as 

 follows : — Consider a carbon molecule or small 

 congerie of molecules just set free by the high 

 temperature from the negative leg of the incan- 

 descent carbon horseshoe. This small carbon 

 mass finds itself in the electrostatic field between 

 the branches of the incandescent carbon con- 

 ductor (see Fig. 14). It is acted ujjon induc- 

 tively, and if it behaves like the hot iron ball in 

 Prof. Guthrie's experiment it loses its positive 

 charge. The molecule then being charged nega- 

 tively is repelled along the lines of electric force against the positive 

 leg. The forces moving it are electric forces, and the repetition of this 

 action would cause a torrent of negatively-charged molecules to pour 

 across from the negative to the positive side of the carbon horseshoe. 

 If we place a metal plate in their path, which is in conducting con- 



I- 



Eough diagram illus- 

 tratiDg a theory of 

 the manner in which 

 projected carbon 

 molecules may ac- 

 quire a negative 

 charge. 



* " On a New Relation between Electricity and Heat," Phil. Mag. vol. xlv. 

 p. 308. 1873. 



