THE FOUR EXPERIMENTAL METHODS. 471 



other in their variations, that the one is cause and the 

 other effect. The same thing may, and indeed must 

 happen, supposing them to be two different effects of 

 a common cause : and by this method alone it would 

 never be possible to ascertain which of the two sup- 

 positions is the true one. The only way to solve the 

 doubt would be that which we have so often adverted 

 to, viz., by endeavouring to ascertain whether we can 

 produce the one set of variations by means of the 

 other. In the case of heat, for example, by increasing 

 the temperature of a body we increase its bulk, but 

 by increasing its bulk we do not increase its tem- 

 perature; on the contrary, (as in the rarefaction of air 

 under the receiver of an air-pump,) we generally 

 diminish it : therefore heat is not an effect, but a 

 cause, of increase of bulk. If we cannot ourselves 

 produce the variations,, we must endeavour, though 

 it is an attempt which is seldom successful, to find 

 them produced by nature in some case in which 

 the pre-existing circumstances are perfectly known 

 to us. 



It is scarcely necessary to say, that in order to 

 ascertain the uniform concomitance of variations in 

 the effect with variations in the cause, the same 

 precautions must be used as in any other case of the 

 determination of an invariable sequence. We must 

 endeavour to retain all the other antecedents un- 

 changed, while that particular one is subjected to the 

 requisite series of variations ; or in other words, that 

 we may be warranted in inferring causation from 

 concomitance of variations, the concomitance itself 

 must be proved by the Method of Difference. 



It might at first appear that the Method of Con- 

 comitant Variations assumes a new axiom, or law of 

 causation in general, namely, that every modification 



