six.] EXPERIMENT. 423 



find some effect, we cannot tell whether the effect is due 

 to one or the other condition, or to both jointly. A second 

 reason is that if no effect ensues we cannot safely conclude 

 that either of them is indifferent ; for the one may have 

 neutralised the effect of the other. In our symbolic logic 

 AB -|- Ab was shown to be identical with A (p. 97), so 

 that B denotes a circumstance which is indifferently 

 present or absent. But if B always go together with 

 another antecedent C, we cannot show the same inde- 

 pendence, for ABC -|- Abe is not identical with A and 

 none of our logical processes enables us to reduce it to A. 



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 

 remarks, 1 " 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 complicated 

 be carefully removed." It has also been well said by 

 Cuvier 2 that the method of physical inquiry consists in 

 isolating bodies, reducing them to their utmost simplicity, 

 and 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 impossible 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 changes of temperature, &c. The electro-magnet is 



1 Lavoisier's Chemistry, translated by Kerr, p. 103. 

 * Cuvier'a Animal Kingdom, introduction, op i, 2. 



