268 INDUCTION. 



Stream running into a reservoir at one end tends to fill it higher and high- 

 er, while a drain at the other extremity tends to empty it. Now, in such 

 cases as these, even if the two causes which are in joint action exactly an- 

 nul 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 afterward. Each agent produces the same amount of 

 effect as if it had acted separately, though the contrary effect which was 

 taking place during the same time obliterated it as fast as it was produced. 

 Here, then, are two causes, producing by their joint operations an effect 

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

 ly, 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 Avhat is commonly called their difference, but which 

 is in reality the result of the addition of opposites ; a conception to which 

 mankind are indebted for that admirable extension of the algebraical cal- 

 culus, which has so vastly increased its powers as an instrument of discov- 

 eiy, by introducing into its reasonings (with the sign of subtraction pre- 

 fixed, and under the name of Negative Quantities) every description what- 

 ever of jjositive phenomena, provided they are of such a quality in reference 

 to those previously introduced, that to add the one is equivalent to sub- 

 tracting an equal quantity of the other. 



There is, then, one mode of the mutual interference 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 agencies which are brought to- 

 gether 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, not a larger amount of liquid, but a solid mass. 



§ 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 heteroge- 

 neous to them — between laws Avhich work together without alteration, and 

 laws which, when called upon to work together, cease and give place to oth- 

 ers — 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 principle of the Composition of Causes ; none 

 that have not some laws which are rigidly fulfilled in every combination 

 into which the objects enter. The weight of a body, for instance, is a 

 pi'operty which it retains in all the combinations in which it is placed. 

 The Aveight of a chemical compound, or of an organized body, is equal to 

 the sum of the weights of the elements which compose it. The weight 

 either of the elements or of the compound will vary, if they be carried far- 

 ther from their centre of attraction, or brought nearer to it ; but whatever 

 effects tlie one effects the other. They always remain precisely equal. So, 

 again, the component parts of a vegetable or animal substance do not lose 

 their mechanical and chemical properties as separate agents, when, by a 

 peculiar mode of juxtaposition, they, as an aggregate whole, acquire physi- 

 ological or vital properties in addition. Those bodies continue, as before, 



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

 of pressure, in diminishing the flow of water through the drain ; which evidently in no way 

 affects the truth or applicability of the principle, since when the two causes act simultaneously 

 the conditions of that diminution of pressure do not arise. 



