ON RA DIA NT HE A T. 59 



number of impulses imparted, in one second, to every eye 

 which sees it, is 577 millions of millions; while the num- 

 ber of impulses imparted, in the same time, by the second 

 band is 000 millions of millions. We may project upon a 

 white screen the beautiful stream of green light from 

 which 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 dis- 

 qualify the vapor, when acting freely as such, from being 

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

 example, consists of hot aqueous vapor. It is scarcely 

 visible in the air of this room, and it would be still less 

 visible if we could burn 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 temper- 

 ature of the oxyhydrogen 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 dif- 

 ferent from its own. 



Thus far we have fixed our attention on atoms and 

 molecules in a state of vibration, and surrounded by a 

 medium which accepts their vibrations, and transmits 

 them through space. But suppose the waves generated by 

 one system of molecules to impinge upon another system, 

 how will the waves be affected? Will they be stopped, or 

 will they be permitted to pass? Will they transfer their 

 motion to the molecules on which they impinge, or will 



