DISCOVERY BY TESTING HYPOTHESES. 511 



hottest, Sir W. Herschel, in the year 1800, examined the 

 solar spectrum with a thermometer, and discovered that 

 the hottest rays were not in the visible spectrum at all, 

 but were just beyond its red extremity. 



Faraday assumed the hypothesis that a wire conveying 

 an electric current would charge a neighbouring wire with 

 static electricity by induction, and tried in the year 1825 

 to verify it by means of experiment. Wheatstone, as- 

 suming the passage of electricity to occupy time, devised 

 and made an experiment in the year 1834 for testing it, 

 * by catching in a mirror, whilst revolving on a horizontal 

 axis at the rate of 800 times in a second, three electrical 

 sparks produced by the discharge of an electrical jar in an 

 interrupted circuit, the interruptions being at each end 

 and in the middle of the conducting wire. In this ex- 

 periment the centre spark fell out of the line of the other 

 sparks by half a degree of the circle,' through which the 

 mirror had revolved in the interval. 1 Coulomb sought to 

 verify his hypothesis of universal magnetism during the 

 year 1802, but found all bodies point axially with regard 

 to the poles of a magnet. 2 Tyndall, also, in 1 856, by devis- 

 ing and making suitable experiments to test the hypothe- 

 sis of diamagnetic polarity, discovered that diamagnetised 

 bodies, like paramagnetised ones, exhibit that property. 



Cavendish discovered in ] 784 that when common air 

 is used to support the combustion of inflammable air (i.e. 

 hydrogen), a dew is formed in the apparatus, and inferred 

 that ' almost all the inflammable air, and one-fifth of the 

 common air, are turned into pure water ; ' and stated that 

 his ' experiments were made principally with a view to 

 find out the cause of the diminution which common air is 

 well known to suffer, by all the various ways in which it 



1 Sir W. S. Harris, Rudimentary Electricty, p. 123. 



2 Ibid., pp. 1, 2, 56. 



