302 VOLTAIC ELECTRICITY. SECT. XXIX. 



found that the positive terminal or pole is hotter than the 

 negative. 



According to Mr. Joule, the quantity of heat generated in a 

 unit of time is proportional to the strength of the current, and 

 when a galvanic current is employed in chemical analysis, the 

 heat in the entire circuit generated in a unit of time is equal to 

 the work expended in producing it, minus that employed in the 

 analysis. In fact, a current of electricity cannot pass through a 

 homogeneous conductor without generating heat in overcoming 

 resistance, an effect proved by Mr. Joule to be proportional to 

 the square of the force of the current, and the same in whatever 

 direction the current may be flowing. Any other thermal action 

 that can take place must depend upon the heterogeneousness of 

 the circuit, and must be reversible with the current. For ex- 

 ample, if a semicircle of bismuth be joined to a semicircle of anti- 

 mony, an electric current in passing through it produces cold 

 where it passes from the bismuth to the antimony by absorption, 

 and heat where it passes from the antimony to the bismuth. 



The transit of the electricity from pole to pole is accompanied 

 by light, and in consequence of the continuous current sparks 

 occur every time the contact of the wires is either broken or re- 

 newed; but considerable intensity is requisite to enable the 

 electricity to force its way through atmospheric air or gas. Both 

 its length and colour are affected by the density of the medium 

 through which it passes. If the medium be gradually rarefied 

 the discharge increases from a spark to a luminous glow, differ- 

 ing in colour in different gases, but white in air. When very 

 much attenuated a discharge may be made to pass across 6 or 7 

 feet of space, while in air of the ordinary density it will not pass 

 through an inch. In rarefied gas it resembles the Aurora by its 

 continuous flashes. When the battery is powerful the luminous 

 effects are very brilliant. 



The most splendid artificial light known is produced by 

 fixing pencils of charcoal at the extremities of the wires, and 

 bringing them into contact. This light is the more remark- 

 able as it is independent of combustion, since the char- 

 coal suffers no apparent change, and, likewise, because it is 

 equally vivid in such gases as do not contain oxygen. It 

 depends upon the molecular arrangement of the charcoal ; for 

 Mr. Grove observes that " carbon in a transparent crystalline 



