NATURE 
[Oct. 8, 1885 
RADIANT LIGHT AND HEAT? 
Ill. (Continued) 
Absorption—Terrestrial Applications. 
ET us next consider the absorption spectra of sub- 
stances, that is to say, the absorption lines which 
substances at ordinary temperatures produce in the 
spectrum of light from a high temperature source, such as 
the sun or the electric arc. This absorption may either 
be general or selective ; it may be spread over a large 
portion of the spectrum, or it may act specially over a 
very limited district or line. It is in the latter case that 
we derive most advantage by studying absorption spectra, 
and there are many substances which may be known at a 
glance by means of their peculiar absorption. Professor 
Stokes has shown, for instance, that blood may at once be 
distinguished from other solutions of similar tint by means 
of the characteristic dark bands which it produces. By 
means of a spectrum microscope Mr. Sorby thinks that 
the thousandth part of a grain of blood may be detected, 
and the same observer asserts that wines of different 
vintages can easily be distinguished from one another in 
the same way. It thus appears that the absorption 
spectrum may in many cases furnish us with an efficient 
and simple means of ascertaining adulteration, for the 
presence of inferior substances which escape detection by 
the taste or sight will at once be recognised when spectrum 
analysis is employed. Russell, Gladstone, Abney, Festing, 
and others have studied with much success the spectra of 
solid and liquid bodies. 
The absorption spectra of gases and vapours at low 
temperatures have been studied by various physicists, and 
amongst them by Janssen, Roscoe, Schuster, and Locwyer. 
Brewster, as we have seen, was the first to observe 
the effect produced on the solar spectrum by nitrous acid 
gas; other gases have since been tried in the same way, and 
many of these give out channelled or fluted absorption 
lines similar to those given out by nitrous acid gas. 
In fine, various researches lead us to conclude that 
gases, and more especially vapours, are in a state of greater 
molecular complexity at a low than at a high temperature, 
for at a low temperature they have a prominent absorp- 
tion for many kinds of rays, whereas at a high temperature 
they have no such strong absorptive power, but absorb 
and radiate only a few definite spectral lines. 
This simplification produced in spectra by the rise of 
temperature has been greatly insisted on by Lockyer, and 
will again come under our review when we have discussed 
the celestial applications of spectrum analysis. 
Meanwhile, I cannot do better than quote the words of 
Lockyer in his Treatise on the Spectroscope and its 
applications (NATURE Series) :—“ We may state generally 
(says that observer) that beginning with one element in 
its most rarified condition, and then following its spectrum 
as the molecules come nearer together, so as at last to 
reach the solid form, we shall find that spectrum become 
more complicated as this approach takes place, until at 
last a continuous spectrum is reached.” 
Before concluding this division of my subject, it will be 
necessary to allude to the absorptive effect produced by 
the earth’s atmosphere on the light and heat of the sun. 
This is a point of great practical as well as scientific 
importance, more especially if we reflect that the atmo- 
sphere is a covering of variable composition, and that the 
variable element (aqueous vapour) is one which no doubt 
exercises a large absorptive influence upon the rays of the 
sun. But there is another element of variability in the 
sun itself, for we more than suspect that the amount of 
radiant energy which we receive from our luminary 
depends to some unknown extent upon the state of his 
surface, and may thus be different in years characterised 
by a maximum number of sun spots, and in years charac- 
terised by a small number of these phenomena. An 
* Continued from p 425. 
additional complication is introduced by the suspicion that 
one of these causes of variability may react upon the other in 
such a way that in those years when the radiation of the 
sun is intrinsically most powerful (if there be such) an ab- 
normally large amount of aqueous vapour may be dis- 
solved in the air, so that we may have on such occasions 
an increased absorption as well as a large intrinsic radia- 
tion, and the one of these causes may thus, to a great 
extent, cover or conceal the other. 
Bearing these points in mind, I shall divide my remarks 
into two sections. I shall treat, 2 the first place, of the 
means which we have at our disposal for estimating the 
whole amount of radiant energy which reaches us from 
the sun at any station, whether this be near the level of the 
sea or at an elevation above it. 
In the second place, 1 shall allude to the means we 
have at our disposal for estimating the amount of any 
one kind of radiant energy that reaches us from the sun. 
An instrument by means of which we may ascertain 
the amount of the sun’s radiant energy is called an 
Actinometer. 
I have recently suggested such an instrument for 
measuring the heating effect of the sun, which has been 
tried at various stations, and appears to work well. It 
consists of a thick hollow cube of brass, surrounded with 
felt, and then again with a covering of polished brass. 
Into the interior of this chamber a suitable thermometer 
is inserted, its bulb being exactly in the centre. There is 
a small hole in one of the sides, through which the heat 
of the sun condensed by a lens is made to fall upon the 
bulb of the thermometer, the instrument having a motion 
in altitude and azimuth so as to enable it to catch the 
sun readily. The exposure is made for a definite time, as 
given by a good chronometer. 
Instruments of this kind have been established in 
various places and at various elevations, and we shall 
certainly be able to derive from them information of im- 
portance as regards the meteorology of the place. To 
what extent we shall be able by their means to separate 
between the two apparent causes of solar variability, 
namely, that due to an intrinsic change in the sun itself, 
and that due toa change in the constitution of the earth’s 
atmosphere, is perhaps an open question. It may be 
hoped that such an instrument may at least enable us 
to advance the problem, even if it prove insufficient to 
bring it to a complete solution. 
Again, Professor Sir Henry Roscoe has invented an 
instrument intended to record the effect of the sun in 
blackening chloride of silver. He is able to prepare a 
paper of a standard sensitiveness, which, by an auto- 
matic arrangement, is exposed for known intervals of 
time. This is an instrument from which we shall no 
doubt obtain valuable information, more especially as the 
more refrangible rays of the sun play an important part 
in terrestrial economy. Still, however, it does not at first 
sight escape the objection above mentioned, or enable us 
to discriminate between the two apparent causes of solar 
variability—the celestial and the atmospheric. 
It has been remarked by the Solar Physics Com- 
mittee, in their report to the British Government (page 
65) that by comparing with a standard certain def- 
nite regions of the solar spectrum, unabsorbed by any 
of the constituents of the earth’s atmosphere, we might 
be able to ascertain any variation in the quantity or 
in the quality of the true solar radiation. This leads 
me to inquire what means we have at present of 
estimating the amount of any particular kind of ray 
which we receive from the sun. In the first place, 
we have the recent extension by Captain Abney of the 
powers of photography, in virtue of which it is not too 
much to assert that we can now obtain a complete map of 
the solar spectrum, with its absorption lines extending 
greatly beyond the visible spectrum on either side. 
We have also the invention and successful construc- 
