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246 
SPECTROSCOPE AND ITS 
APPLICATIONS: 
IV. 
N what has now been stated we first saw Newton 
founding spectral analysis, by using a hole in a shutter 
and a prism ; then we discussed Wollaston’s substitution 
of the slit; after that Mr. Simms’ introduction of the 
collimating lens was referred to ; and then the growth of 
the modern spectroscope. 
It is time, now, that we came to the applications of the 
instrument. And in dealing with these applications I 
shall divide my subject into two perfectly distinct portions. 
I shall first deal with those which depend upon the diffe- 
rent modes in which light is given out or radiated by 
. ON THE 
Fic. 
9 - Electric Lamp. 
various bodies under different physical conditions, with, in 
fact, the vadiation of light. And, in the second place, I 
shall deal with the spectroscope’s story of the way in 
which white light giving a continuous spectrum is stopped 
or absorbed by different transparent bodies—with in fact 
the absorption of light. 
The first application of this question of radiation is one 
cf the most general importance. It enables us to differ- 
entiate between solid, liquid, and gaseous substances, and 
between gaseous or vaporous substances in different 
Stages of pressure, If, for instance, we take a platinum 
wire and heat it to redness, and examine by means of the 
spectroscope, the light emitted we shall find that only red 
rays are visible, then if the wire be gradually heated more 
strongly, the yellow, green, and blue, rays will become 
visible, until finally when the wire has attained a brilliant 
white heat, the whole of the colours of the spectrum will 
NA1IURE 
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be present, If I were to burn a piece of paper, or a match 
or an ordinary coal gas flame, you all know we should get. 
a white light, but you may possibly not all know that if 
we raise any solid or liquid to a state of incandescence or 
glowing heat we should get exactly that same sort of light, 
which will always give us a continuous spectrum. Before 
a large audience the best method of showing this fact is 
to use an apparatus called the electric lamp, and to pass 
the current of electricity through two carbon points, 
which are intensely heated by their resistance to the pas- 
sage of the current. The spectrum obtained from these 
points, by means of the dispersion of two bisulphide of 
carbon prisms, is quite continuous from end to end. Now 
carbon is a solid, and therefore if we take carbon as an 
example of a solid or liquid substance in a state of vivid 
incandescence, and we obtain from these carbon points a 
continuous spectrum, you must accept that as an indica- 
Fic. 20.—Arrangement of the electric lamp used for rapid comparisons. 
tion of the truth of what I say, for I have not time to ex- 
periment on every solid and every liquid substance.» The 
spectrum is received on the screen, and you see it is con- 
tinuous, that is to say, there are no breaks, such as those 
we saw in the figure representing a portion of the solar 
spectrum on page 167 where the black lines represent the 
breaks in the solar spectrum which are called the Fraiin- 
hofer lines. 
Let us then consider this fact established, namely, that 
solid or liquid bodies, when heated to a vivid incan- 
descence, give a continuous sprectrum without bright 
lines. Under these circumstances the light to the eye, 
without the spectroscope, will be white, like that of a 
white hot poker ; if the degree of incandescence is not 
so high, the light may only be red, like that of a red- 
hot poker. But so far as the spectrum goes—and it will 
expand towards the violet, as the incandescence increases, 
as before stated— it will be continuous, 
Now, suppose, instead of giving you the spectrum of 
eee 
[ Fan. 30, 1873. 
» 
