CHAP, iv.] ELECTRICITY AND MAGNETISM. 239 



gas, and it will also cause chemical combination to take 

 place with explosion, when passed through detonating 

 mixtures of gases. Thus equal volumes of chlorine and 

 hydrogen are exploded by the spark. So are oxygen and 

 hydrogen gases, when mixed in the proportion of two 

 volumes of the latter to one of the former. Even the 

 explosive mixture of common coal gas mixed with from 

 four to ten times its own volume of common air, can be 

 thus detonated. A common experiment with the so- 

 called electric pistol consists in filling a small brass vessel 

 with detonating gases and then exploding them by a 

 spark. The spark discharge is sometimes applied to 

 the firing of blasts and mines in military operations, a 

 small quantity of fulminating powder being placed in 

 the path of the spark to kindle the larger charge of 

 gunpowder or other explosive. (See also Art. 370.) 



287. Physiological Effects. The physiological 

 effects of the current have been described in Lesson 

 XIX. Those produced by the spark discharge are more 

 sudden in character, but of the same general nature. 

 The bodies of persons killed by the lightning spark 

 frequently exhibit markings of a reddish tint where the 

 discharge in passing through the tissues has lacerated or 

 destroyed them. Sometimes these markings present a 

 singular ramified appearance, as though the discharge 

 had spread in streams over the surface at its entry. 



288. Calorific Effects. The flow of electricity 

 through a resisting medium is in every case accompanied 

 by an evolution of heat. The laws of heating due to 

 currents are given in Art. 367. The disruptive discharge 

 is a transfer of electricity through a medium of great 

 resistance and accompanied by an evolution of heat. 

 A few drops of ether in a metallic spoon are easily 

 kindled by an electric spark. The spark from an electric 

 machine, or even from a rubbed glass rod, is hot enough 

 to kindle an ordinary gas-jet. In certain districts of 

 America, during the driest season of the year, the mere 



