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LXX. Observations on the Lines of the Solar Spectrum, and 
on those produced by the Earth's Atmosphere, and by the 
Action of Nitrous Acid Gas. By Sir Davin Brewster, 
K.H., V.P.R.S. Ed.* 
[NX a paper on the Monochromatic lamp, &c., read before 
the Royal Society of Edinburgh on the 15th April 1822, 
and published in their Transactions, I recorded some of my 
earliest experiments on the action of coloured media on the 
solar spectrum. ‘These experiments were continued at irre- 
gular intervals, with the view of obtaining distinguishing cha- 
racters of coloured media, of investigating the cause of the 
colours of natural bodies, and of examining more correctly 
the phzenomena of the overlapping colours of equal refrangi- 
bility, which I had announced in the paper already referred 
to. The results to which I was conducted on the two last of 
these subjects have been already published, in two papers, one 
on the analysis of solar light, and the other on the colours of 
natural bodies. 
The first and the principal object of my inquiries, namely, 
the discovery of a general principle of chemical analysis, in 
which simple and compound bodies might be characterized 
by their action on definite parts of the spectrum, still re- 
mained to be pursued. ‘The coloured juices of plants—arti- 
ficial salts and their solutions, and various glasses and mi- 
nerals—had afforded me many beautiful examples of this 
species of action; and after determining the locality of these 
actions in reference to Fraunhofer’s principal lines, and their 
intensity, as depending on the thickness of the absorbing me- 
dium and the brightness of the spectrum, I was able to di- 
stinguish all such compounds, by merely looking through 
them at a well-formed spectrum. Even in those cases where 
the eye could recognise no difference between the colours of 
two substances that exercised different specific actions upon 
light, their discrimination was instantly effected by viewing 
them through a standard coloured medium. 
As some of these bodies attacked the spectrum at fwo, 
three, four, and even five or more points at once, it became 
probable that the number and intensity of such actions de- 
pended on the number and nature of the elements which en- 
tered into the composition of the body, or, what is nearly the 
same thing, that it was the sum of all the separate actions of 
such elements; and hence the next step in the inquiry was, 
to determine the action of elementary bodies on the solar 
spectrum. ‘This inquiry was not limited to coloured bodies, 
* From the Transactions of the Royal Society of Edinburgh, corrected. 
