EXAMPLES OF THE EXPLANATION OF LAWS. 535 



which present the conditions necessary for bringing that law 

 into action ; a process fertile in demonstrations of special laws 

 previously unsuspected, and explanations of others already 

 empirically known. 



For instance, Faraday discovered by experiment, that 

 voltaic electricity could be evolved from a natural magnet, 

 provided a conducting body were set in motion at right angles 

 to the direction of the magnet : and this he found to hold 

 not only of small magnets, but of that great magnet, the earth. 

 The law being thus established experimentally, that electricity 

 is evolved, by a magnet, and a conductor moving at right 

 angles to the direction of its poles, we may now look out for 

 fresh instances in which these conditions meet. Wherever a 

 conductor moves or revolves at right angles to the direction 

 of the earth's magnetic poles, there we may expect an evolu- 

 tion of electricity. In the northern regions, where the polar 

 direction is nearly perpendicular to the horizon, all horizontal 

 motions of conductors will produce electricity; horizontal 

 wheels, for example, made of metal; likewise all running 

 streams will evolve a current of electricity, which will circulate 

 round them; and the air thus charged with electricity may be 

 one of the causes of the Aurora Borealis. In the equatorial 

 regions, on the contrary, upright wheels placed parallel to the 

 equator will originate a voltaic circuit, and waterfalls will 

 naturally become electric. 



For a second example; it has been proved, chiefly by 

 the researches of Professor Graham, that gases have a 

 strong tendency to permeate animal membranes, and diffuse 

 themselves through the spaces which such membranes in- 

 close, notwithstanding the presence of other gases in those 

 spaces. Proceeding from this general law, and reviewing a 

 variety of cases in which gases lie contiguous to membranes, 

 we are enabled to demonstrate or to explain the following 

 more special laws : 1st. The human or animal body, when 

 surrounded with any gas not already contained within the 

 body, absorbs it rapidly ; such, for instance, as the gases of 

 putrefying matters : which helps to explain malaria. 2nd. The 

 carbonic acid gas of effervescing drinks, evolved in the stomach, 



