PHYSICAL BASIS OF SOLAR CHEMISTRY, 259 
The experiment justifying this conclusion is now for 
the first time to be made before a public audience. -I pass 
a beam through our two prisms, and the spectrum spreads 
its colors upon the screen. Between the lamp and the 
prism I interpose a snapdragon light. Alcohol and 
water are here mixed with common salt, and the metal 
dish that holds them is heated by a spirit-lamp. The 
vapor from the mixture ignites and we have a mono- 
chromatic flame. Through this flame the beam from the 
lamp is now passing; and observe the result upon the 
spectrum. You see a shady band cut out of the yellow 
not very dark, but sufficiently so to be seen by everybody 
present. 
But let me exalt this effect. Placing in front of the 
electric lamp the intense flame of a large Bunsen's burner, 
a platinum capsule containing a bit of sodium less than a 
pea in magnitude is plunged into the flame. The sodium 
soon volatilizes and burns with brilliant incandescence. 
The beam crosses the flame, and at the same time the 
yellow band of the spectrum is clearly and sharply cut out, 
a band of intense darkness occupying its place. On with- 
drawing the sodium, the brilliant yellow of the spectrum 
takes its proper place, while the reiutroduction of the 
flame causes the band to reappear. 
Let me be more precise: The yellow color of the spec- 
trum extends over a sensible space, blending on one side 
with the orange and on the other with the green. The 
term "yellow band" is therefore somewhat indefinite. 
This vagueness may be entirely removed. By dipping the 
carbon-point used for the positive electrode into a solution 
of common salt, and replacing it in the lamp, the bright 
yellow band produced by the sodium vapor stands out 
from the spectrum. When the sodium flame is caused to 
act upon the beam it is that particular yellow band that 
is obliterated, an intensely black streak occupying its 
place. 
An additional step of reasoning leads to the conclusion 
that if, instead of the flame of sodium alone, we were to 
introduce into the path of the beam a flame in which 
lithium, strontium, magnesium, calcium, etc., are in a 
state of volatilization, each metallic vapor would cut out a 
system of bands, corresponding exactly in position with 
the bright bauds of the same metallic vapor. The 
