432 ACTUAL WORKING IN ORIGINAL RESEARCH. 



same time being a definite quantity. Effects vary in 

 amount with their causes ; for instance, the difference of 

 level of the mercury in a barometer diminishes as the 

 pressure of the air upon the mercury becomes less. The 

 greatness of an effect proves the greatness of its real cause. 

 "We never know that we have found the entire cause or 

 effect until we can show an equivalent of the one for each 

 equivalent of the other; for instance, in a chemical 

 analysis we cannot be certain that we have found all 

 the constituents of a compound until we have adequately 

 accounted for the entire weight of substance taken for the 

 analysis. When several causes conspire to produce an 

 effect, the value of each should be separately and definitely 

 measured by excluding one at a time. Many residual 

 phenomena have been discovered by this method ; for 

 instance, the planet Neptune was found by means of a 

 prior observation, of a residuary disturbance of Uranus, 



6 In Sir Humphry Davy's experiments upon the decom- 

 position of water by galvanism, it was found that besides 

 the two components of water, oxygen and hydrogen, an 

 acid and alkali were developed at the two opposite poles of 

 the machine. As the theory of the analysis of water did 

 not give reason to expect these products, they were a 

 residual phenomenon, the cause of which was still to be 

 found. Some chemists thought that electricity had the 

 power of producing these substances of itself ; and if their 

 erroneous conjecture had been adopted, succeeding re- 

 searches would have gone upon a false scent, considering 

 galvanic-electricity as a producing rather than a decom- 

 posing force. The happier insight of Davy conjectured 

 that there might be some hidden cause of this portion of 

 the effect; the glass containing the water might suffer 

 partial decomposition, or some foreign matter might be 

 mingled with the water, and the acid and alkali be dis- 



