PHILOSOPHY OF INDUCTIVE INFERENCE. 273 



with my assumed piece. But in trying to identify iron, I 

 am always open to mistake. Nor is this liability to mis- 

 take a matter of speculation only v . 



The history of chemistry shows that the most confident 

 inferences may have been falsified by the confusion of one 

 substance with another. Thus strontia was never discri- 

 minated from baryta until Klaproth and Haiiy detected 

 differences between some of their properties x . - Accordingly 

 chemists must often have inferred concerning strontia 

 what was only true of baryta, and vice versd. There is 

 now no doubt that the recently discovered substances, 

 caesium and rubidium were long mistaken for potassium?. 

 Other elements' have often been confused together, for 

 instance, tantalum and niobium ; sulphur and selenium ; 

 cerium, lanthanum, and didymium ; yttrium and erbium. 



Even the best-established laws of physical science do 

 not exclude false inference. No law of nature has been 

 better established than that of universal gravitation, and 

 we believe with the utmost confidence that any body 

 capable of affecting the senses will attract other bodies, 

 and fall to the earth if not prevented. Euler remarks 

 that, although he had never made trial of the stones 

 which compose the church of Magdeburg, yet he had 

 not the least doubt that all of them were heavy, and 

 would fall if unsupported. But he adds, that it would 

 be extremely difficult to give any satisfactory explanation 

 of this confident belief 2 . The fact is, that the belief ought 

 not to amount to certainty until the experiment has been 

 tried, and in the meantime a slight amount of uncer- 



v Professor Bowen has excellently stated this view. ' Treatise on 

 Logic.' Cambridge, U.S.A., 1866. P. 354. 



x "Whewell's 'History of the Inductive Sciences,' vol. iii. p. 174. 



y Roscoe's 'Spectrum Analysis/ ist edit. p. 99. 



z Euler's ' Letters to a German Princess,' translated by Hunter, 

 and ed. vol. ii. pp. 17-18. 



T 



