548 SPECIAL METHODS OF DISCOVERY. 



of solar rays by means of a thermometer, W. Herschel, in 

 the year 1800, discovered the dark heat rays of the solar 

 spectrum. Fizeau successfully employed a revolving mirror 

 for determining the velocity of light. Melloni adopted 

 the method of using a thermo-electric pile and galvano- 

 meter as a measure of radiant heat, and made an extensive 

 series of determinations of the relative degrees of trans- 

 parency of different solid and liquid bodies to rays of heat 

 6y means of it. Tyndall also employed a very delicate 

 method of determining the relative degrees of thermic 

 opacity of vapours and gases by means of the same instru- 

 ments, and evolved many new truths. By applying a steady 

 pull to the ends of wires whilst raising them to and cool- 

 ing them from a temperature of redness, I discovered a 

 sudden molecular change in iron. By enclosing the wire 

 in a glass tube provided with a small exit-pipe contain- 

 ing water, Professor Barrett discovered that the molecular 

 change during cooling was attended by a sudden evolu- 

 tion of heat; and Dr. Norris, by devising and using a 

 third and different mode of manipulation, has recently 

 been enabled to investigate systematically in a different 

 way this anomalous phenomenon. 



d. By making converse experiments to those already 

 known. This plan is based upon the mechanical principle 

 of action and reaction, and some very important discoveries 

 have been made by its means. An electric current having 

 been found by Oersted to produce magnetism, Faraday 

 adopted this method, and, by a converse form of Oersted's 

 experiment, discovered the way to produce electric currents 

 by means of magnetism, and laid the foundation of the 

 branch of science called magneto-electricity, and all its 

 great technical appliances. Peltier's discovery of electro- 

 thermacy was also effected by means of converse forms of 

 experiment to those employed by Seebeck to discover 



