266 
NATURE 

ancient and modern doctrine regarding atoms. Allow me to read 
from that article one other short passage finely describing 
the present aspect of atomic theory :—‘‘ The existence of the 
chemical atom, already quite a complex little world, seems very 
probable ; and the description of the Lucretian atom is wonder- 
fully applicable to it. We are not wholly without hope that the 
real weight of each such atom may some day be known—not 
merely the relative weight of the several atoms, but the number 
in a given volume of any material ; that the form and motion of 
the parts of each atom and the distances by which they are 
separated may be calculated ; that the motions by which they 
produce keat, electricity, and light may be illustrated by exact 
geometrical diagrams; and that the fundamental properties of 
the intermediate and possibiy constituent medium may be arrived 
at. Then the motion of planets and music of the spheres will 
be neglected for a while in admiration of the maze in which the 
tiny atoms run.” 
Even before this was written some of the anticipated results 
had been partially attained. Loschmidt in Vienna had shown, 
and not much Jatter Stoney independently in England showed, 
how to reduce from Clausius and Maxwell’s kinetic theory of 
gases a superior limit to the number of atoms in a given mea- 
surable space. I was unfortunately quite unaware of what 
Loschmidt and Stoney had done when I made a similar estimate 
en the same foundation, and communicated to NATURE in an 
article on ‘* The Size of Atoms.” But questions of personal 
priority, however in eresting they may be to the persons con- 
cerned, sink into insignificance in the prospect of any gain of 
deeper insight into the secrets of nature. The triple coincidence 
of independent reasoning in this case is valuable as confirmation 
of a conclusion violently contravening ideas and opinions which 
had been almost universally held regarding the dimensions of 
the molecula: structure of matter. Chemists and other naturalists 
had been in the habit of evading questions as to the hardness or 
indivisibility of atoms by virtually assuming them to be infinitely 
small and infinitely numerous. We must now no longer look 
upon the atom, with Boscovich, as a mystic point endowed with 
inertia and the attribute of attracting cr repelling other such 
centres with forces depending upon the intervening distances (a 
supposition only tolerated with the tacit assumption that the 
inertia and attraction of each atom is infinitely small and the 
number of atoms infinitely great), nor can we agrce with those 
who have attributed to the atom occupation of space with in- 
finite harduess and strength (incredible in any finite body) ; but 
we mut realise it as a piece of matter of measurable dimensions, 
with shape, motion, and Jaws of action, intelligible subjects of 
scientific investigation. 
The prismatic analysis of light discovered by Newton was 
estimated by himself as being ‘‘ihe oddest, if not the most con- 
siderable, detection which hath hitherto been made in the opera- 
tions of nature.” 
Had he not been deflected from the subject, he could not have 
failed to obtain a pure spectrum ; but this, with the inevitably 
consequent discovery of the dark lines, was reserved for the nine- 
teenth century. Our fundamental knowledge of the dark lines 
is due solely 10 Fraunhofer. Wollaston saw them, but did not 
discover them. Brewster laboured long and well to perfect the 
prismatic analysis of sunlight ; and his observations on the dark 
bands produced by the absorption of interposed gases and vapours 
laid important foundations for the grand superstructure which he 
scarcely lived to see. Piazzi Smyth, by spectroscopic observation 
performed on the Peak of Teneriffe, added greatly to our know- 
ledge of the dark lines produced in the solar spectrum by the 
absorption of our own atmosphere. The prism became an insiru- | 
ment for chemical qualitative analysis in the hands of Fox Talbot 
and Herschel, who first showed how, through it, the old ‘* blow- 
pipe test’’ or generally the estimation of substances from the 
colours which they give to flames, can be prosecuted with an 
accuracy and a discriminating power not to be attained when the 
colour 1s judged by the unaided eye. But the application of this 
test to solar and stellar chemistry had never, I believe, been sug- 
gested, either directly or indirectly, by any other naturalist, when 
Stokes taught it to me in Cambridge at some time prior to the 
summer of 1852. The observational and experimental founda- 
tions on which he built were :— 
1. The discovery by Fraunhofer of a coincidence between his 
double dark Ine D of the solar spectrum and a double bright 
line which he observed in the spectra of ordinary artificial 
flames. 
2. A very rigorous experimental test of this coincidence by 

| a cup. 
Prof. W. H. Miller, which showed it to be accurate to an asto- 
nishing degree of minuteness. 
3. The fact that the yellow light given out when salt is thrown 
on burning spirit consists almost solely of the two nearly iden- 
tical qualities which constitute that double bright line. 
. Observations made by Stokes himsel!, which showed the 
bright line D to be absent in a candle-flame when the wick was 
snuffed clean, so ‘as not to project ‘into the luminous envelope, 
and from an alcohol flame when the spirit was burned in a watch- 
glass. And 
5. Foucault’s admirable discovery (L’Institut, Feb. 7, 1849) 
that the Voltaic arc between charcoal points is ‘fa medium 
which emits the rays D on its own account, and at the same time 
absorbs them when they come from another quarter.” 
The conclusions, theoretical and practical, which Stokes taught 
me, and which I gave regularly afterwards in my public lectures 
in the University of Glasgow, were: 
1. That the double line D, whether bright or dark, is due to 
vapour of sodium. 
2. That the ultimate atom of sodium is susceptible of regular 
elastic vibrations, like those of a tuning-fork or of stringed 
musical instruments ; that like an instrument with two strings 
tuned to approximate unison, or an approximately circular elastic 
disc, it has two fundamental notes or vibrations of approximately 
equal pitch; and that the periods of these vibrations are pre- 
cisely the periods of the two slightly different yellow lights 
constituting the double bright line D. 
3. That when vapour of sodium is at a high enough tempe- 
rature to become itself a source of light, each atom executes these 
two fundamental vibrations simultaneously ; and that therefore 
the light proceeding from it is of the two qualities constituting 
the double bright line D. 
4. That when vapour of sodium is present in space across 
which light from another source is propagated, its atoms, ac- 
cording to a well-known general principle of dynamics, are set to 
vibr. te in either or bo h of those fundamental modes, if some of 
the incident light is cf one or other of their periods, or some of 
one and some of the other ; so that the energy of the waves of 
those particular qualities of light is converted into thermal vibra- 
tions of the medium and dispersed in all directions, while light 
ofall other qualities, even though very nearly agreeing with them, 
is transmitted with comparatively no loss, 
5. That Fraunhofer’s double dark line D of solar and stellar 
spectra is due to the presence of vapour of sodium in atmospheres 
surrounding the sun and those stars in whose spectra it had been 
observed. 
6, That other vapours than sodium are to be found in the 
atmospheres of sun and stars by searching for substances pro- 
ducing in the spectra of artificial flames bnght lines coinciding 
with other dark lines of the solar and stellar spectra than the 
Fraunhofer line D. 
The last of these propositions I felt to be confirmed (it was 
perhaps partly suggested) by a striking and beautiful experiment 
admirably adapted for lecture illustrations, due to Foucault, which 
bad been shown to me by M. Duboscque Soleil, and the Abbé 
Moigno, in Paris in the month of October 1850. A prism and 
lenses were arranged to throw upon a screen an approximately 
pure spectrum of a vertical electric arc between charcoal poles 
of a powerful battery, the lower one of which was hollowed like 
When pieces of copper and pieces of zinc were sepa- 
rately thrown into the cup, the spectrum exhibited, in perfectly 
definite positions, magnihcent well-marked bands of different 
colours characteristic of the two metals. When a piece of brass, 
compounded of copper and zinc, was put into the cup, the 
spectrum showed all the bands, each precisely in the place in 
which it had been seen when one metal or the other had been 
used separately. 
It is much to be regretted that this great generalisation was not 
published to the world twenty years ago. I say this, not because 
it is to be regretted that Angstrom should have the credit of hav- 
ing in 1853 published independently the statement that an ‘‘in- 
candescent gas emits luminous rays of the same refrangibility as 
those which it can absorb” ; or that Balfour Stewart should have 
been unassisted by it when, coming to the subject from a yery 
different point of view, he made, in his extension of the ‘* Theory 
of Exchanges,” * the still wider generalisation that the radiating 
power of every kind of substance is equal to its absorbing power 
for every kind of ray ; or that Kirchhoff also should have in 1859 
independently discovered the same proposition, and shown its 
* Edin, Transactions, 1858-59. 
[ Aug. 3; 1871 
sd axcptteaabibig ~~ 

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