212 INDUCTION. 



more causes, which, if they acted separately, would produce effects 

 contrary, or at least conflicting with each other ; one of them tending 

 to undo, wholly or partially, what the other tends to do. Thus, the 

 expansive force of the gases generated by the ignition of gunpow- 

 der tends to project a bullet towards the sky, while its gravity tends to 

 make it fall to the ground. A stream running into a reservoir at one 

 end tends to fill it higher and higher, while a drain at the other exJn-emity 

 tends to empty it. Now, in such cases as these, even if the two causes 

 which are in joint action exactly annul one another, still the laws of 

 both are fulfilled; the effect is the same as if the drain' had been open 

 for half an hour first,* and the stream had flowed in for as long after- 

 wards. Each agent produced the same amount of -effect as if it had 

 acted separately, though the contrary effect which was taking plafce 

 during the same time obliterated it as fast as it was produced. Here, 

 then, we have two causes, producing by their joint operation an effect 

 which at first seems quite dissimilar to those which they produce sep- 

 arately, 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 commonly called their dif- 

 ference, 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 in- 

 creased its powerxS as an instrument of discovery, by introducing into 

 its reasonings (with the sign of subtraction prefixed, and under the 

 name of Negative Quantities) every description whatever 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 interference of laws of na- 

 ture in which, even when the eoncuiTent 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 agen- 

 cies 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, in- 

 stead 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 ease in which it is 

 heterogeneous to them ; between laws which work together without 

 alteration, and laws which, when called upon to work together, cease 

 and give place to others ; is one of the fundamental distinctions in 

 nature. The former case, that of the Composition of Causes, is the 

 general one; the other is always special and exceptional. There are 

 no objects which do not, as to some of their phenomena, obey the prin- 

 ciple of the Composition of Causes ; none that have not some laws 

 which are rigidly fulfilled in eveiy combination into which the objects 

 enter. The weight of a body, for instance, is a property which it 

 retains in all the combinations in which it is placed. The weight of a 

 chemical compound, or of an organized body, is equal to the sum of 



* I omit, for simplicity, to take into account the effect, in this latter case, of the diminu- 

 tion of pressure, in diminishing the flow of the water through the drain ; which evidently 

 iu no way allects the truth or applicability of the principle. 



