86 FRAGMENTS OF SCIENCE 



wMch these bands were derived. This luminous stream is 

 the incandescent vapor of silver. The rates of vibration 

 of the atoms of that vapor are as rigidly fixed as those of 

 two tuning-forks; and to whatever height the temperature 

 of the vapor may be raised, the rapidity of its vibrations, 

 and consequently its color, which wholly depends upon 

 that rapidity, remain unchanged. 



The vapor of water, as well as the vapor of silver, has 

 its definite periods of vibration, and these are such as to 

 disqualify the vapor, when acting freely as such, from be- 

 ing raised to a white heat. The oxy hydrogen flame, for 

 example, consists of hot aqueous vapor. It is scarcely vis- 

 ible in the air of this room, and it would be still less visi- 

 ble if we could bum the gas in a clean atmosphere. But 

 the atmosphere, even at the summit of Mont Blanc, is 

 dirty; in London it is more than dirty; and the burning 

 dirt gives to this flame the greater portion of its present 

 light. But the heat of the flame is enormous. Cast iron 

 fuses at a temperature of 2,000° Fahr. ; while the tempera- 

 ture of the oxy hydrogen flame is 6,000° Fahr. A piece of 

 platinum is heated to vivid redness, at a distance of two 

 inches beyond the visible termination of the flame. The 

 vapor which produces incandescence is here absolutely 

 dark. In the flame itself the platinum is raised to daz- 

 zling whiteness, and is even pierced by the flame. When 

 this flame impinges on a piece of lime we have the daz- 

 zling Drummond light. But the light is here due to the 

 fact that when it impinges upon the solid body, the vibra- 

 tions excited in that body by the flame are of periods 

 different from its own. 



Thus far we have fixed our attention on atoms and 

 molecules in a state of vibration, and surrounded by a 



