468 INDUCTION. 



of Difference what effect was due to the body, when 

 the remainder which was due to the heat would be 

 given by the Method of Residues. But we can do 

 none of these things ; and without them the appli- 

 cation of any of the three methods to the solution of 

 this problem would be illusory. It would be idle, for 

 instance, to attempt to ascertain the effect of heat by 

 subtracting from the phenomena exhibited by a body, 

 all that is due to its other properties ; for as we have 

 never been able to observe any bodies without a 

 portion of heat in them, the effects due to that heat 

 may form a part of the very results, which we affect to 

 subtract in order that the effect of heat may be shown 

 by the residue. 



If, therefore, there were no other methods of 

 experimental investigation than these three, we should 

 be for ever unable to determine the effects due to heat 

 as a cause. But we have still a resource. Though 

 we cannot exclude an antecedent altogether, we may 

 be able to produce, or nature may produce for us, 

 some modification in it. By a modification is here 

 meant, a change in it, not amounting to its total 

 removal. If some modification in the antecedent A 

 is always followed by a change in the consequent a, 

 the other consequents b and c remaining the same ; 

 or, vice versd, if every change in a is found to have 

 been preceded by some modification in A, none being 

 observable in any of the other antecedents ; we may 

 safely conclude that a is, wholly or in part, an effect 

 traceable to A, or at least in some way connected with 

 it through causation. For example, in the case of 

 heat, though we cannot expel it altogether from any 

 body, we can modify it in quantity, we can increase 

 or diminish it ; and doing so, we find by the various 

 methods of experimentation or observation already 



