QUANTITATIVE INDUCTION. 129 



simple proportion to the mass of each of the attracting 

 bodies, the mass being measured by, or proportional to 

 inertia. Similarly, in all cases of ' direct unimpeded 

 action/ as Sir J. Herschel has remarked x , we may expect 

 simple proportion to manifest itself. In such cases the 

 equation expressing the relation may have the still 

 simpler form y = mx. 



A similar simple relation holds true wherever there 

 is a conversion of one substance or form of energy into 

 another. The quantity of chloride of silver is propor- 

 tional to the quantity either of chlorine or silver. The 

 amount of heat produced in friction is exactly propor- 

 tional to the mechanical energy absorbed. It was ex- 

 perimentally proved by Faraday that ' the chemical 

 power of the current of electricity is in direct proportion 

 to the quantity of electricity which passes.' When an 

 electric current is produced, the quantity of electric 

 energy is simply proportional to the weight of metal 

 dissolved. If electricity is turned into heat, there is 

 again simple proportion. Wherever, in fact, one thing 

 is but another thing with a new aspect, we may expect to 

 find the law of simple proportion. It is only among the 

 most elementary causes and effects that this simple re- 

 lation will hold true. Simple conditions do not, generally 

 speaking, produce simple results. The planets move in 

 approximate circles round the sun, but the apparent 

 motions, as seen from the earth, are so various, that men 

 have not believed in such a simple view of the matter 

 for more than about two centuries and a half. All those 

 motions, again, are summed up in the law of gravity, 

 of no great complexity, yet men never have, and never 

 can be, able to exhaust the complications of action and 

 reaction, even among a small number of planets. We 

 should be on our guard against a tendency to assume that 

 * 'Preliminary Discourse/ &c. p. 152. 

 K 



