490 INDUCTION. 



actual production of the phenomenon are most frequently in- 

 accessible to our observation. If we could not have learned 

 the composition of water until we found instances in which it 

 was actually produced from oxygen and hydrogen, we should 

 have been forced to wait until the casual thought struck some 

 one of passing an electric spark through a mixture of the two 

 gases, or inserting a lighted taper into it, merely to try what 

 would happen. Besides, many substances, though they can 

 be analysed, cannot by any known artificial means be recora- 

 pounded. Further, even if we could have ascertained, by the 

 Method of Agreement, that oxygen and hydrogen were both 

 present when water is produced, no experimentation on oxygen 

 and hydrogen separately, no knowledge of their laws, could 

 have enabled us deductively to infer that they would produce 

 water. We require a specific experiment on the two com- 

 bined. 



Under these difficulties, we should generally have been 

 indebted for our knowledge of the causes of this class of effects, 

 not to any inquiry directed specifically towards that end, but 

 either to accident, or to the gradual progress of experimenta- 

 tion on the different combinations of which the producing 

 agents are susceptible ; if it were not for a peculiarity belong- 

 ing to effects of this description, that they often, under some 

 particular combination of circumstances, reproduce their 

 causes. If water results from the juxtaposition of hydrogen 

 and oxygen whenever this can be made sufficiently close and 

 intimate, so, on the other hand, if water itself be placed in 

 certain situations, hydrogen and oxygen are reproduced from 

 it : an abrupt termination is put to the new laws, and the 

 agents reappear separately with their own properties as at 

 first. What is called chemical analysis is the process of 

 searching for the causes of a phenomenon among its effects, 

 or rather among the effects produced by the action of some 

 other causes upon it. 



Lavoisier, by heating mercury to a high temperature in a 

 close vessel containing air, found that the mercury increased 

 in weight, and became what was then called red precipitate, 

 while the air, on being examined after the experiment, proved 



