SEARCHING FOR ONE THING AND FINDING ANOTHER. 519 



mercury by electro-chemical action. 1 On another occasion 

 he was searching for magneto-electric induction by sudden 

 change of temperature of a magnet, and* heated a wire of 

 iron to redness whilst it was stretched by an elastic band 

 at its end ; and, on allowing the wire to cool, he observed 

 a peculiar motion of the band, and, by further research, 

 discovered that iron, whilst cooling from a red heat and 

 under longitudinal strain, undergoes a sudden molecular 

 and magnetic change, attended by increase of length, at a 

 particular temperature. 2 



All these instances, and many more which might be 

 given, prove the truth of the * fundamental laws of disco- 

 very,' already given, 3 and show that we have only to place 

 matter or its forces under new conditions, or observe them 

 in a new aspect, take a sufficient number of instances, and 

 employ sufficiently delicate means of observation, and we 

 must make new discoveries, and that this is really an 

 infallible method. 



6. By assuming the truthfulness and certainty of all 

 the great principles of science* This is a most valuable 

 method. We might, for example, assume that wherever a 

 substance suffers a powerful magnetic or other molecular 

 change of its mass or surface, all the other properties of its 

 mass or surface respectively are simultaneously more or 

 less affected, and then invent and make suitable experi- 

 ments, to test that hypothesis. After I had observed 

 the anomalous molecular change which occurs in a 

 stretched wire of iron or steel whilst cooling from a full 

 red heat, Professor Barrett discovered the simultaneous 

 evolution of heat which accompanies that change ; and I 

 have no doubt that various other accompanying changes 



1 See Proceedings of the Royal Society, vol. xii. p. 217. 



- Ibid. 1869, No. 108, pp. 260, 265. 



8 See p. 458. * See Chapter XIV. 



