EXAMPLES OP THE FOUR METHODS. 453 



possible, when we find that not only one substance, but many 

 substances, possess the capacity of acting as antidotes to 

 metallic poisons, and that all these agree in the property of 

 forming insoluble compounds with the poisons, while they 

 cannot be ascertained to agree in any other property what- 

 soever. We have thus, in favour 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 Differ- 

 ence ; the evidence of which, though it never can amount to 

 that of the Method of Difference properly so called, may ap- 

 proach 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 adja- 

 cent 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 con- 

 ductor itself. Near and around the positive prime conductor 

 there is negative electricity, and near and around the negative 

 prime conductor there is positive electricity. When pith balls 

 are brought near to either of the conductors, they become elec- 

 trified with the opposite electricity to it; either receiving a 

 share from the already electrified atmosphere 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 are in opposition ; or, if withdrawn in their electrified state, 



* For this speculation, as for many other of my scientific illustrations, 

 I am indebted to Professor Bain, of Aberdeen, who has since, in his profound 

 treatises entitled "The Senses and the Intellect," and "The Emotions and the 

 Will," carried the analytic investigation of the mental phenomena according 

 to the methods of physical science, to the most advanced point which it has yet 

 reached, and has worthily inscribed his name among the successive constructors 

 of an edifice to which Hartley, Brown, and James Mill had each contributed 

 their part. 



