COMPOSITION OF CAUSES. 267 



upon first by one of the two forces, and afterward by the other. This law 

 of nature is called, in dynamics, the principle of the Composition of Forces ; 

 and in imitation of that well-chosen expression, I shall give the name of the 

 Composition of Causes to the principle which is exemplified in all cases in 

 which the joint effect of several causes is identical with the sum of their 

 separate effects. 



This principle, however, by no means prevails in all departments of the 

 jfield of nature. The chemical combination of two substances produces, as 

 is well known, a third substance, with properties different from those of 

 either of the two substances separately, or of both of them taken together. 

 Not a trace of the properties of hydrogen or of oxygen is observable in 

 those of their compound, water. The taste of sugar of lead is not the 

 sum of the tastes of its component elements, acetic acid and lead or its 

 oxide ; nor is the color of blue vitriol a mixture of the colors of sulphuric 

 acid and copper. This explains why mechanics is a deductive or demon- 

 strative science, and chemistry not. In the one, w'e can compute the ef- 

 fects of combinations of causes, whether real or hypothetical, from the 

 laws which we know to govern those causes when acting separately, be- 

 cause they continue to observe the same laws when in combination which 

 they observe when separate: whatever would have happened in conse- 

 quence of each cause taken by itself, happens when they are together, 

 and we have only to cast up the results. Not so in the phenomena which 

 are the peculiar subject of the science of chemistiy. There most of the 

 uniformities to w liich the causes conform when separate, cease altogether 

 when they are conjoined ; and we are not, at least in the present state of 

 our knowledge, able to foresee what result will follow from any new com- 

 bination until we have tried the specific experiment. 



If this be true of chemical combinations, it is still more true of those far 

 more complex combinations of elements which constitute organized bodies ; 

 and in which those extraordinary new uniformities arise which are called 

 the laws of life. All organized bodies are composed of parts similar to 

 those composing inorganic nature, and which have even themselves existed 

 in an inorganic state ; but the phenomena of life, which result from the 

 juxtaposition of those parts in a certain manner, bear no analogy to any 

 of the effects which would be produced by the action of the component 

 substances considered as mere physical agents. To whatever degree we 

 might imagine our knowledge of the properties of the several ingredients 

 of a living body to be extended and perfected, it is certain that no mere 

 summing up of the separate actions of those elements will ever amount to 

 the action of the living body itself. The tongue, for instance, is, like all 

 other parts of the animal frame, composed of gelatine, fibrine, and other 

 products of the chemistry of digestion ; but from no knowledge of the 

 properties of those substances could we ever predict that it could taste, un- 

 less gelatine or fibrine could themselves taste ; for no elementary fact can 

 be in the conclusion which was not in the premises. 



There are thus two different modes of the conjunct action of causes ; 

 from which arise two modes of conflict, or mutual interference, between 

 laws of nature. Suppose, at a given point of time and space, two or 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 gunpowder tends to project a bullet 

 toward the sky, while its gravity tends to make it fall to the ground. A 



