DISCOVERY BY MEANS OF NEW ELECTRICAL INSTRUMENTS. 479 



their photospheres differ in composition from that of 

 the sun. It was also by the aid of the spectroscope that 

 Huggins, in 1864, discovered that some of the nebulae are 

 really gaseous, by finding that instead of giving dark lines 

 upon a bright ground, they showed a few faintly-luminous 

 ones on a dark ground, exactly as highly-heated lumi- 

 nous gases do ; and thus proved the hypothesis suggested 

 by Sir W. Herschel, about the year 1786. And quite 

 recently, Crookes, by the invention of his radiometer, has 

 been enabled to discover the rotation of bodies by the 

 influence of heat. 



Galileo was one of the first to make and employ a crude 

 kind of thermometer, by means of which, in its more im- 

 proved form, so many new truths have been found. In 

 the early forms of that instrument, air was employed ; a 

 Dutchman, named Drebbel, introduced spirits of wine 

 instead ; and in the year 1670 mercury began to be used. 

 In 1693, Halley, by means of a thermometer, discovered 

 that the temperature of boiling- water was a fixed one ; 

 and in 1714, Daniel Gabriel Fahrenheit, of Dantzig, 

 invented his thermometer with fixed points of tempera- 

 ture upon it. The invention and use of the steam-engine 

 also led to many new experiments and discoveries respect- 

 ing the nature and relations of steam and of heat. Watt, 

 in 1764-65, made a systematic series of experiments to 

 determine the pressure of steam at different temperatures 

 above the boiling-point. By a discovery made by Melloni, 

 during an investigation of the transparency of bodies to 

 rays of heat, that rock-salt was extremely transparent to such 

 rays, we were supplied with the means of concentrating, 

 refracting, and dispersing those rays ; and Tyndall was 

 thereby enabled to discover the degree of thermic tran- 

 sparency of the atmosphere and of numerous gases and 

 vapours. 



