14 
trum was illustrated by the spectrum of incandescent ime. The 
fact that the spectrum of a glowing vapour had bright bands or 
lines, each vapour having its characteristic lines, was strikingly 
illustrated by an experiment with the optical lantern. A short 
Bunsen burner was suitably adjusted in the lantern—into its flame 
a pellet of sodium was introduced, on the screen at once appeared 
the yellow band so well known to spectroscopists as the sodium 
line, or lines, or the D line, or lines, in Fraunhofer’s chart. The 
characteristic bright limes of lithium and strontium were shown to 
a few of the audience by means of the large direct vision spectro- 
scope. Subsequently, using an induction coil and introducing into 
the electric circuit a Leyden phial, the spectra of lead and thallium 
were shown in the spectroscope. The lecturer proceeded to state 
and illustrate another principle of spectrum analysis. An incan- 
descent solid or liquid body shining through absorbent vapours 
gave a continuous spectrum crossed by dark lines or hands these 
lines having in each instance the same position as the bright lines 
of the spectrum of the vapour,—given the same temperature and 
pressure In 1859 Kirchoff discovered that when the vapour of 
sodium was interposed between the slit and solar spectrum the D 
line was darkened. Jt was found that vapours absorb the same 
colours which they radiate or emit when heated to incandescence. 
This was the explanation of Fraunhofer’s lines. When an incan- 
descent body was surrounded by glowing vapours, one of three 
three things, the lecturer said, would happen ; if the body and the 
vapour were at the same temperature there would be no result, as 
emission and absorption would be equal, if the temperature of the 
glowing vapour were higher than that of the body, the result would 
be bright lines, if lower, dark or absorption hands. By the intro- 
duction of a pellet of sodium about the size of a pea into the flame 
of a Bunsen burner in front of the slit the experiment known as 
the reversal of the sodium line was very successfully performed. 
The glowing sodium vapour at a lower temperature than the in- 
candescent lime producing the spectrum on the screen, was crossed 
by a black absorption band. By the application of these principles, 
sodium, iron, hydrogen, and many other substances had been dis- » 
covered in the sun. Sereen projections of the spectra of various 
specimens of human blood in different conditions, and of the solar 
prominences were also exhibited. Spectrum analysis, the lecturer 
said, was the most subtle and far-reaching of all analytical methods. 
Other methods were refined and gave results of the greatest ac- 
curacy, but this transcended all others. Swan found that the lines 
of sodium are shown by the spectroscope when only one 2,500.000th of 
a grain of sodium was used. New metals, thallium indium, cesium 
and rubidium had been discovered by the spectroscope. Since the 
