EXAMPLES OF THE FOUR METHODS. 



269 



sence of one single circumstance. 

 Now, in the preceding argument, they 

 differ in the presence or absence not 

 of a single circumstance, but of a single 

 substance ; and as every substance has 

 innumerable properties, there is no 

 knowing what number of real differ- 

 ences are involved in what is nomi- 

 nally and apparently only one differ- 

 ence. It is conceivable that the 

 antidote, the peroxide of iron, for 

 example, may counteract the poison 

 through some other of its properties 

 than that of forming an insoluble 

 compound with it ; and if so, the 

 theory would fall to the ground, so 

 far as it is supported by that instance. 

 This source of uncertainty, which is a 

 serious hindrance to all extensive 

 generalisations in chemistry, is how- 

 ever reduced in the present case to 

 almost the lowest degree possible, 

 when we find that not only one sub- 

 stance, but many substances, possess 

 the capacity of acting as antidotes to 

 metallic poisons, and that all these 

 agree in the property of forming in- 

 soluble compounds with the poisons, 

 while they cannot be ascertained to 

 agree in any other property whatso- 

 ever. We have thus, in favour of the 

 theory, all the evidence which can be 

 obtained by what we termed the In- 

 direct Method of Difference, or the 

 Joint Method of Agreement and Dif- 

 ference ; the evidence of which, though 

 it never can amount to that of the 

 Method \ of Difference properly so 

 called, may approach indefinitely near 

 to it. 



§ 2, Let the object be * to ascertain 

 the law of what is termed induced 

 electricity ; to find under what condi- 

 tions any electrified body, whether 

 positively or negatively electrified, 

 gives rise to a contrary electric state 

 in some other body adjacent to it. 



The most familiar exemplification 



■* For this speculation, as for many other 

 of my scientific illustration*, I am indebted 

 to Professor Bain, whose subsequent trea- 

 tise on Logfic abounds with apt illustra- 

 tions of all the inductive methods, 



of the phenomenon to be investigated 

 is the following. Around the prime 

 conductors of an electrical machine, 

 the atmosphere to some distance, or 

 any conducting surface suspended in 

 that atmosphere, is found to be in an 

 electric condition opposite to that of 

 the prime conductor itself. Near and 

 around the positive prime conductor 

 there is negative electricity, and near 

 and around the negative prime con- 

 ductor there is positive electricity. 

 When pith balls are brought near to 

 either of the conductors, they become 

 electrified with the opposite electri- 

 city to it ; either receiving a share 

 from the already electrified atmos- 

 phere by conduction, or acted upon 

 by the direct inductive influence of 

 the conductor itself ; they are then 

 attracted by the conductor to which 

 they are in opposition ; or, if with- 

 drawn in their electrified state, they 

 will be attracted by any other oppo- 

 sitely charged body. In like manner 

 the hand, if brought near enough to 

 the conductor, receives or gives an 

 electric discharge. Now we have no 

 evidence that a charged conductor 

 can be suddenly discharged unless by 

 the approach of a body oppositely 

 electrified. In the case, therefore, of 

 the electric machine, it appears that 

 the accumulation of electricity in an 

 insulated conductor is always accom- 

 panied by the excitement of the con- 

 trary electricity in the surrounding 

 atmosphere, and in every conductor 

 placed near the former conductor. It 

 does not seem possible, in this case, 

 to produce one electricity by itself. 



Let us now examine all the other 

 instances which we can obtain re- 

 sembling this instance in the given 

 consequent, namely, the evolution of 

 an opposite electricity in the neigh- 

 bourhood of an electrified body. As 

 one remarkable instance we have the 

 Leyden jar; and after the splendid 

 experiments of Faraday in complete 

 and final establishment of the sub- 

 stantial identity of magnetism and 

 electricity, we may cite the magnet, 

 both the natural and the electrq- 



