THERMOMETRIC SCALES. 75 



balls of electroscopes is adopted as a standard 

 test of electric excitation. The extent of mechan- 

 ical action is therefore the standard test of the 

 excitation of both heat and electricity. Various 

 scales of degrees of expansion are adopted in ther- 

 mometers for measuring the extent of vibration 

 of molecules produced by heat. Reaumur's ther- 

 mometer is graduated into eighty equal parts of a 

 glass tube, ranging between the freezing and boil- 

 ing points of water, with a minus scale of degrees 

 extended below the freezing point. The ther- 

 mometer invented by Celsius has 100 between 

 the freezing and boiling points, with a similar 

 minus scale. Fahrenheit, to obtain a greater 

 range of degrees, adopted the extremely low 

 temperature produced by a frigorific mixture of 

 snow, or ice, and salt, as a zero point, with a grad- 

 uation of 32 to the ordinary freezing point of 

 water, and 212 to the boiling point. 



Different kinds of substances have each a 

 peculiar extent of molecular vibration, and con- 

 sequent expansion. Mercury, being the most uni- 

 form between the freezing and boiling points, is 

 selected for use in the bulbs of thermometers. 



VIBRATION OF THE ELECTRIC ETHER EXCITES VIBRA- 

 TION IN PARTICLES OF ALL MATTER. 



The particles of all kinds of bodies solid, liq- 

 uid, and aeriform are made to vibrate by heat, 

 and transmit action by means of the electric me- 



