EXPERIMENT. 31 



If we want to prove that oxygen is necessary to life, we 

 must not put a rabbit into a vessel from which the oxygen 

 has been exhausted by a burning candle. We should then 

 have not only an absence of oxygen, but an addition of 

 carbonic acid, which may have been the destructive agent. 

 For a similar reason Lavoisier avoided the use of atmo- 

 spheric air in experiments on combustion, because air was 

 not a simple substance, and the presence of nitrogen might 

 impede or even alter the effect of oxygen. As Lavoisier 

 expressly remarks f , ' In performing experiments, it is a 

 necessary principle, which ought never to be deviated 

 from, that they be simplified as much as possible, and that 

 every circumstance capable of rendering their results com- 

 plicated be carefully removed.' It has also been well said 

 by CuvierS that the method of physical inquiry consists in 

 isolating bodies, reducing them to their utmost simplicity, 

 and in bringing each of their . properties separately into 

 action, either mentally or by experiment. 



The electro-magnet has been of the utmost service in 

 the investigation of the magnetic properties of matter, by 

 allowing of the production or removal of a most powerful 

 magnetic force without disturbing any of the other ar- 

 rangements of the experiment. Many of Faraday's most 

 valuable experiments would have been frustrated had it 

 been necessary to introduce a heavy permanent magnet, 

 which could not be suddenly moved without shaking the 

 whole apparatus, disturbing the air, producing currents 

 by differences of temperature, &c. The electro-magnet is 

 perfectly under control, and its influence can be brought 

 into action, reversed, or stopped by merely touching a 

 button. Thus Faraday was enabled to prove the rotation 

 of the plane of circular polarized light by the fact that a 

 certain light ceased to be visible when the electric current 



f Lavoisier's 'Chemistry/ translated by Kerr, p. 103. 

 " Cuvier's 'Animal Kingdom, 1 introduction, pp. i, 2. 



