294 INDUCTION, 



soluble, and therefore incapable of combining with the tissues, although to 

 the extent of its solubility it has a medicinal influence, though an entirely 

 different class of organic actions. 



The preceding instances have afforded an induction of a high order of 

 conclusiveness, illustrative of the two simplest of our four methods ; though 

 not rising to the maximum of certainty which the Method of Difference, 

 in its most perfect exemplification, is capable of affording. For (let us 

 not forget) the positive instance and the negative one which the rigor of 

 that method requires, ought to differ only in the presence or absence of 

 one single circumstance. Now, in the preceding argument, they differ in 

 the presence or absence not of a single circumsta7ice, but of a single sub- 

 stance : and as every substance has innumerable properties, there is no 

 knowing what number of real differences are involved in what is nominally 

 and apparently only one difference. 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 hinderance to 

 all extensive generalizations in chemistry, is, however, reduced in the pres- 

 ent case to almost the lowest degree possible, when we find that not only 

 one substance, but many substances, possess the capacity of acting as anti- 

 dotes to metallic poisons, and that all these agree in the property of form- 

 ing insoluble compounds with the poisons, while they can not be ascer- 

 tained to agree in any other property whatsoever. We have thus, in favor 

 of the theory, all the evidence which can be obtained by what we termed 

 the Indirect Method of Difference, or the Joint Method of Agreement and 

 Difference ; 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 conditions 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 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 electric- 

 ity to it ; either receiving a share from the already electrified atmosphere 

 by conduction, or acted upon by the direct inductive influence of the con- 

 ductor itself : they are then attracted by the conductor to which they are 

 in opposition ; or, if withdrawn in their electrified state, they will be at- 

 tracted by any other oppositely charged body. In like manner the hand, 

 if brought near enough to the conductor, receives or gives an electric dis- 

 charge; now we have no evidence that a charged conductor can be sud- 

 denly discharged unless by the approach of a body oppositely electrified. 



* For this speculation, as for many other of my scientific illustrations, I am indebted to 

 Professor Bain, whose subsequent treatise on Logic abounds with apt illustrations of all the 

 inductive methods. 



