INDUCTION AND DEDUCTION. 259 



non or process, that they postulate oue another, or that one is depend- 

 ent on the other ; but this is merely an idea having no real basis for its 

 support, but simply a perception which may or may not arise in the 

 mind of any one. 



Aristotle denoted induction as being the passage from liarticulars to 

 generals, since, in physical inquiry-, our first concern is with the knowl- 

 edge of the phenomenon and afterward with its explanation ; but in this 

 sense it is clear that he regarded induction not as a method, but as a rule 

 of investigation. 



It is plain that if all the forces of nature and their laws were known 

 to us, if we knew all things in their nature, action, and properties, the 

 investigation of a particular process and its explanation would be a sim- 

 ple deductive problem ; each single case would then be soluble through 

 a conclusion of the understanding. Suppose, for instance, that the rust- 

 ing of iron in the air were the point to be explained ; the i^revious exam- 

 ination of rust has determined that it contains iron, oxygen, and water; 

 the comi:)osition of air is also known 5 the elements, therefore, for ex- 

 plaining the process of rusting are before us, but further inquiry shows 

 us that iron in oxygen in the presence of the vapor of water does not 

 rust ; there must, then, be some constituent principle of the air, besides 

 oxygen and vapor, in order that iron should undergo the process of 

 rusting ; now we know that the air actually contains a very small portion 

 of carbonic acid, and examination shows that a mere trace of carbonic 

 acid suffices, with sufficient access of oxygen, to convert a large mass of 

 iron into oxide; but the rust itself contains no carbonic acid. The 

 question then is : What part does this acid play in the process ? Another 

 known fact now suffices to complete the explanation ; this is the action 

 of the oxidulated carbonate of iron ; in damp air it attracts oxygen and 

 is converted into the higher oxide which enters not into combination with 

 carbonic acid ; it is by the rusting of the metal that the lower oxide first 

 originates, and this combines with carbonic acid, which through the 

 passage of the oxidulate into free oxide becomes capable of exerting in 

 a hundredfold degree its original action on the metal, so that gradually 

 the whole i^iece is throughout converted into iron-rust. Inquiry further 

 establishes that there is a special case where iron in damp air, even 

 without the i^resence of carbonic acid, is thus rusted, when the air, 

 namely, contains ammonia; but that in that case the rust does not 

 extend, and that, lastly, an electrical process is cooperative with the 

 rusting. 



To this class of investigations belongs also that of the production of 

 dew by Dr. Wells. That dew is a watery precipitate produced by 

 refrigeration admitted of no doubt, nor that the modes of refrigeration 

 were only two. The problem only turned upon the question, whether 

 the conditions of the cooling were dependent on conduction or on radi- 

 ation, which point was susceptible of determination by experiments 

 guided by known laws. 



