EXAMPLES OF THE EXPLANATION OF LAWS. 283 



principles combined with a third, namely, that the peristaltic contrac- 

 tion acts easily upon substances in a state of solution. The negative 

 general proposition, that animal substances do not absorb these salts, 

 oonti-ibutes to the explanation by accounting tor the absence of a 

 counteracting cause, namely, absorption by tho stomach, which in the 

 case of oth(?r substances possessed of the requisite chemical properties, 

 interferes to prevent them from reaching the substances whicli they 

 are destined to dissolve. 



§ 4. From the foregoing and similar instances, w6 may see- the im- 

 portance, when a law of nature previously unknown has been brought 

 to light, or when new light has been thrown upon a known law by 

 experiment, of examining all cases which present the conditions neces- 

 sary for bringing that la\V into action ; a process necessarily fertile in 

 demonstrations of special laws previously unsuspected, and explana- 

 tions of others already empirically known. 



■ For instance, Faraday discovered by experiment, that voltaic elec- 

 tricity 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 evolution of electricity. In the northei'n 

 regions, where the polar direction is nearly perpendicular to the hori- 

 zon, all horizontal motions of conductors will produce electricity ; hori- 

 zontal 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 

 tlie Aurora BoroaKs. 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 recently been found, chiefly by the 

 researches of Professor Graham, that gases have a strong tendency to 

 permeate aramal membranes, and diffuse themselves through tlie spaces 

 which such membranes enclose, notwithstanding the presence of other 

 gases in those spaces. Proceeding from this general law, and review- 

 ing 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. 2d. The carbonic acid gas of effervescing drinks, evolved 

 in tlio stomacli, permeates its membranes, and rapidly spreads tlirough 

 the system, where, as suggested in a former note, it probably combines 

 with tlie iron contained in the blood. ^ 3d. Alcohol taken into the 

 stomach (the temperature of tho stf)Tnach is above the boiling point of 

 pure alcohol,) passes into vapor and sprfiads through the system with 

 great rapidity; (which combined with the high combustibility of alco- 

 hol, or in other words its ready combination with oxygen, may perhaps 

 help to explain the bodily warmth immediately consequent on drinking 



