HEAT. 149 



mo-pile, a few drops of water and sulphuric acid about 

 equal parts : the heat evolved will illustrate the origin 

 of heat from chemical reaction. 



A few crystals of nitrate of ammonium in a test-tube 

 may have an equal bulk of water poured upon them ; 

 the cold produced is from the absorption of heat dur- 

 ing liquefaction. 



Interpose between the source of heat and the ther- 

 mo-pile various things, such as rock-salt, a solution of 

 iodine in bisulphide of carbon, glass, crystals of various 

 kinds, tubes filled with gases and vapors of various 

 sorts. Also, project a solar spectrum with a part of 

 the same beam that projects the galvanometer by the 

 method described upon page 112. Move the thermo- 

 pile through the various colors, and note the degree 

 indicated by the galvanometer, particularly beyond the 

 red end of the spectrum. The thermo-pile should be 

 placed where the Fraunhofer lines are seen best upon 

 a small screen placed temporarily to receive it. 



Many experiments on this subject will be found in 

 Tyndall's work on Heat, which one will find himself 

 able to repeat with satisfaction. 



CALORESCENCE. 



Let the light from \h^ porte lumiere, or from the elec- 

 tric or lime light, be sent through a vessel containing 

 bisulphide of carbon in which some iodine has been 

 dissolved : the solution will be jet black and will stop 

 every light ray, but will permit the rays of greater wave 

 length to freely traverse it. A lens may now be inter- 

 posed and the obscure rays treated in every way like 

 luminous rays. With a very powerful beam platinum 

 foil may be raised to incandescence in the focus of the 



