426 INDUCTION. 



class of phenomena commonly called mechanical, 

 namely the phenomena of the communication of 

 motion (or of pressure, which is tendency to motion) 

 from one body to another. In this important class 

 of cases of causation, one cause never, properly 

 speaking, defeats or 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 mechanical 

 philosophy, 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 

 combination 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 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 colour of green 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 



