DISCOVERY BY MEANS OF REPETITION OF EXPERIMENTS. 543 



only one of the chief steps in the process ; suggestion, 

 imagination, observation, comparison, and inference were 

 also employed, and constituted an important part in 

 the operation. At the same time, a certain increase of 

 knowledge was due in each instance to experiment and 

 observation alone, and so far experiment alone may be 

 regarded as a fertile source of new discoveries. 



a. By making or repeating, in a modified form, 

 experiments suggested by other persons. Sometimes 

 one man suggests an experiment, and another carries it 

 out, or repeats it in a modified form. ' Hooke proposed 

 to observe the vibrations of a bell, by strewing flour upon 

 it. But it was Chladni, a Grerman philosopher, who en- 

 riched acoustics with the discovery of the vast variety of 

 symmetrical figures which are exhibited on plates of 

 regular forms, when made to sound.' l Newton, about 

 the year 1770, suggested, but Lagrange discovered, that 

 the cause of the moon always presenting the same side 

 to the earth, was the attraction of the latter upon the 

 swelling at the lunar equator. Previous astronomers sug- 

 gested, and Adams and Le Verrier confirmed by calcula- 

 tion, that the orbit of the November meteors extended 

 beyond Uranus. Volta's great discovery arose from re- 

 peating Gralvani's experiments and studying the results ; 

 he made his celebrated pile in the year 1800. Wart- 

 mann, in the year 1846, by repeating with rays of heat 

 the experiment which Faraday in 1845 had made with 

 rays of light, discovered the rotation of the plane of 

 polarisation of heat rays by magnetism. By modifying 

 the apparatus employed by Faraday in the liquefaction of 

 gases, I was enabled to subject a large number of solid 

 and liquid substances to the action of liquefied carbonic 

 anhydride, ammonia, cyanogen, and hydrochloric acid 



1 Whewell, History of the Inductive Sciences, 3rd edit. vol. ii. p. 258. 



