EXAMPLES OF THE FOUR METHODS. 485 



insoluble compounds with the poisons, while they 

 cannot be ascertained to agree in any other property 

 whatsoever. 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 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. 



No similar defect of completeness in proof will 

 be found in the following original investigation, for 

 which I am indebted to Mr. Alexander Bain, at present 

 Lecturer on Moral Philosophy in Marischal College, 

 Aberdeen; one of the men from whom science and 

 philosophy have most to hope, and who has permitted 

 me to lay his extensive knowledge of every depart- 

 ment of physical inquiry freely under contribution, 

 for the purpose of exemplifying and illustrating the 

 doctrines of this work. 



3. 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 pheno- 

 menon 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 conductor there is positive 

 electricity. When pith balls are brought near to 



