ON RADIANT 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 oxyhydrogen 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 
