406 INDUCTION. 



frustrates another ; both have their full effect. If a body is 

 propelled in two directions by two forces, one tending to 

 drive it to the north and the other to the east, it is caused 

 to move in a given time exactly as far in both directions as 

 the two forces would separately have carried it; and is left 

 precisely where it would have arrived if it had been acted upon 

 first by one of the two forces, and afterwards 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 field of nature. The chemical combina- 

 tion of two substances produces, as is well known, a third 

 substance with properties entirely different from those of 

 either of the two substances separately, or 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 colour of blue vitriol a mixture of the 

 colours of sulphuric acid and copper. This explains why 

 mechanics is a deductive or demonstrative science, and 

 chemistry not. In the one, we can compute the effects of 

 all combinations of causes, whether real or hypothetical, 

 from the laws which we know to govern those causes when 

 acting separately; because they continue to observe the 

 same laws when in combination which they observed when 

 separate : whatever would have happened in consequence 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 

 chemistry. There, most of the uniformities to which the 

 causes conformed 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 



