COMPOSITION OF CAUSES. 429 



which at first seems quite dissimilar to those which 

 they produce separately, but which on examination 

 proves to be really the sum of those separate effects. 

 It will be noticed that we here enlarge the idea of 

 the sum of two effects, so as to include what is com- 

 monly called their difference, but which is in reality 

 the result of the addition of opposites ; a conception 

 to which, as is well known, mankind are indebted for 

 that admirable extension of the algebraical calculus, 

 which has so vastly increased its powers as an instru- 

 ment of discovery, by introducing into its reasonings 

 (with the sign of subtraction prefixed, and under the 

 name of Negative Quantities) every description what- 

 ever of positive phenomena, provided they are of such 

 a quality in reference to those previously introduced, 

 that to add the one is equivalent to subtracting an 

 equal quantity of the other. 



There is, then, one mode of the mutual inter- 

 ference of laws of nature in which, even when the 

 concurrent causes annihilate each other's effects, each 

 exerts its full efficacy according to its own law, its 

 law as a separate agent. But in the other description 

 of cases, the two agencies which are brought together 

 cease entirely, and a totally different set of phenomena 

 arise : as in the experiment of two liquids which, 

 when mixed in certain proportions, instantly become 

 a solid mass, instead of merely a larger amount of 

 liquid. 



2. This difference between the case in which the 

 joint effect of causes is the sum of their separate effects, 

 and the case in which it is heterogeneous to them ; 

 between laws which work together without alteration, 

 and laws which, when called upon to work toge- 

 ther, cease and give place to others; is one of the 



