Geology and Magnetism. 505 



" Conducting liquids may be heated in a similar manner. This 

 fact may be seen in a great variety of ways : dilute sulphuric acid 

 may be made to boil in a siphon connecting two vessels, in which 

 the poles of an extensive series of batteries are placed. Another 

 mode of showing the same fact is to take a piece of string and 

 moisten it with acid, connecting the extremities with the poles of a 

 series of galvanic batteries, when it will begin to smoke, and be- 

 come charred from the heat produced. 



" The next property which a battery displays is its power of igni- 

 ting metallic or charcoal points when joined to the two ends of the 

 battery, and held so that they barely touch ; a light is then exhibit- 

 ed equal in brilliancy to that of a little sun. The spark seems 

 to depend principally upon a combustion of fine particles of metal, 

 and, when charcoal or hard gas coke is used, upon little points of 

 it flying from one pole to the other ; so that one pole wastes 

 away and the other increases, till the flame becomes quite encased 

 in a mass of carbonaceous matter. This flame is singularly re- 

 pelled or attracted by a magnet held in its vicinity. Heat is, 

 indeed, one of the effects of chemical action ; and though we might 

 by a fallacious reasoning be led to assert that chemical action is the 

 effect of heat, a very slight examination will show the absolute futi- 

 lity of such reasoning. In fact, we have no heat of which the cause 

 is known, but that which is derived from, and proportionate to che- 

 mical action." 



Mr. Hopkins applies this to terrestrial phenomena fur- 

 ther on, and remarks. 



" If we admit the existence of subterranean currents, and that 

 these exert a slow decomposing power, like that of the voltaic bat- 

 tery, we have a sufficient power for our purpose. In the first place, 

 we have a mechanical tension on the consolidated parts of the rocks, 

 by the linear action of the currents passing through them ; and 

 should the intensity of the currents be very great, fractures would 

 ensue, more or less at right angles to the direction of the force. 

 These fractures would admit air and water, and thus produce intense 

 heat, by the avidity with which the metallic nature of the bases of 

 the earths and alkalies combines with the oxygen. 



