522 SPECIAL METHODS OF DISCOVERT. 



d. By assuming that certain general statements 

 which are true of one force or substance are true to 

 some extent of other?. By means of this method, fol- 

 lowed by appropriate investigations, many propositions 

 which are true of ligKt have been found to be true of 

 radiant heat ; for instance, Faraday having, in the year 

 1845, discovered the magnetic rotary polarisation of light, 

 Wartmann, in the following year, succeeded in discover- 

 ing a similar action with a beam of heat. 



In a similar manner, properties known to exist in 

 potassium have been predicted of and found to exist in 

 rubidium : for instance, the carbonates of sodium and 

 potassium are not decomposed by a red heat, neither are 

 those of rubidium or caesium. Some of the statements 

 which are true of chlorine have been found to be true, in 

 varying degrees, of bromine and iodine ; and some of those 

 true of iron have been found to be true of manganese, 

 chromium, and aluminium ; and similarly with every dis- 

 tinct family or group of substances united by similarities. 

 After I had found the molecular change in antimony 

 electro-deposited from its chloride, I sought for and dis- 

 covered it in that deposited from its bromide and iodide ; 

 and after having found irregular magnetic changes in iron 

 by heat, I also found similar ones in nickel. 



The method itself is based upon the fact, that where 

 any particular substance is found to possess a particular 

 property, other substances which are similar to it in most 

 respects may also be reasonably expected to possess it. 

 Sir Humphry Davy having discovered that potassium 

 might be isolated by means of electrolysis, immediately 

 proceeded to isolate sodium and other bodies of analogous 

 properties, and succeeded. 



e. By assuming the existence of converse principles 

 of action. Magneto-electricity and electro-dynamic indue- 



