164 THE ELECTRIC LIGHT. 



in the clouds is the most striking of these effects that is 

 witnessed in nature. The aurora borealis is another of the 

 forms in which electrical light is manifested. In certain 

 states of the atmosphere, also, pencils of light are seen 

 upon pointed objects, such as the tips of the masts and 

 spars of a vessel at sea, and the summits of spires and oth- 

 er projecting points of buildings on land. 



There is a good deal of mystery about some of the forms 

 in which the light produced by electricity appears, but in 

 that which is developed by artificial means for purposes 

 of illumination, it is well ascertained that the effect is due 

 to the incandescence of solid particles by the agency of in- 

 tense heat. It is found that, when an interruption is made 

 in an electrical circuit, heat is developed, provided that the 

 current is powerful enough to force its way across the in- 

 terval. Sometimes the interruption consists of a wire or 

 other conductor too slender to convey the whole current. 

 In this case the wire is at once heated more or less intense- 

 ly, according to the force of the current in relation to the 

 slenderness of the wire. 



This is the way, in fact, in which blasting charges, for 

 example, are often fired in rocks or under water. The car- 

 tridge containing the charge is prepared beforehand by two 

 wires coming in on opposite sides of it, and connected to- 

 gether in the middle of the gunpowder by a very slender 

 tcire, too fine to convey readily the whole electrical charge 

 which is to pass through the circuit. The outer ends of 

 the two side wires are then connected with the electrical 

 battery in such a manner that the charge may at any mo- 

 ment be sent through them. The battery, of course, may 

 be placed at any distance, provided that wires can be laid, 

 or other electrical communications made from the two poles 

 of it to the place where the cartridge is deposited. Then, 

 \vhen the battery is set in operation, and the connections 



