346 
called Czesium. Rubidium was the next metal which was 
discovered in this way. Take another instance, the dis- 
covery of thallium by Mr. Crookes. Mr. Crookes was 
working with a seleniferous deposit from the Hartz 
mountains, when, by the aid of the spectroscope, he dis- 
covered this metal, which, I am informed, is now exten- 
sively used in the manufacture of fireworks. 
The spec- 
Fic. 27.—Side view of Star Spectroscope, showing the arrangement by 
which the light from a spark is thrown into the instrument by means 
of the reflecting prism ¢, by a mirror F. 
trum of this metal is extremely distinct and beautiful, 
and you will understand why it has been named thallium, 
from the Greek word for a twig, on account of the beauti- 
ful green colour of the single line ordinarily visible. 
A fourth element has been discovered by means of the 
spectroscope by two German chemists, Professors Reich 
and Richter, who were experimenting on zinc blend, and 
found two unknown indigo bands in the spectrum, which 
they successfully traced to the existence of a small 
quantity of a new metallic {element, which has been 
named Indium, 
You all know how important chemical analysis is in 
Fic. 28.—Plan of Star Spectroscope._ 1, Eye-piece end of telescope: n, In- 
terior tube, carrying A, cylindrical lense ; D, Slit of spectroscope ; G, 
Collimating lens ; 4, Prisms; Q, Micrometer. 
thousands of things connected with the arts, manufac- 
tures, and commerce, in detecting adulteration for in- 
stance, and in these matters the spectroscope gives our 
chemists a power which was undreamt of a few years ago. 
There is another very beautiful application of the spec- 
troscope which perhaps many of you will say is of more 
practical importance than those I have already brought 
to your notice, You know that, in the Bessemer process 
NATURE 
[Mar. 6, 1873 
five tons of cast iron are turned into cast steel in twenty 
minutes. Now steel is only cast iron minus some carbon. 
It is clear, therefore, that the process depends upon 
getting rid of the carbon. How then can the spectro- 
“scope aid us in determining the time at which the carbon 
is got rid of? Nothing is more easy. The heat from 
the incandescent iron is so intense that the vapour of the 
different substances mixed with it is visible above the 
retort in which the metal is placed, and we get, so to 
speak, an atmosphere of incandescent vapour surround- 
ing the cast iron. When we examine these incandescent 
vapours by means of a spectroscope, it is found that the 
spectrum changes very considerably at different times 
during the combustion of this cast iron. Now it so 
happens, that the process of conversion is such a delicate 
one that a mistake of ten seconds either way spoils the 
whole five tons which are being operated upon. You 
Tic. 29 
Fic 29,—Dircct:vision Star Spectroscope. s 
A, Telescope ; s, Slit; p, Prism plate; &, Observing telescope; M, 
| Micrometer. 
Fic. 32. 
Fig. 30.—Sun Spectroscope. 
| will see in a moment, therefore, that this is a case in 
which any rule-of-thumb or very rough method might 
now and then lead to a mistake ; but when the spectrum 
of these incandescent vapours thrown out by the cast-iron 
| is examined very carefully by means of a spectroscope, it 
is found that at the first the spectrum of carbon is quite 
visible, but at the right moment, which has been found 
by practice, that spectrum disappears, the combustion 
having been sufficient. All we have to do now, to ensure 
the charge being properly turned out, is, therefore, by 
means of the spectroscope, simply to watch certain lines 
in the spectrum, and when theyshow signs of disappearance 
say ‘‘ Now,” and the thing is done without any possibility 
of error. This is an instance of the practical application 
of the spectroscope in one direction ; now let me give 
you one in another. 
When Dr. Bence Jones wished to determine some 
