THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[August 12, 1871. 
m 
Even before tliis was written some of the anticipated 
results had been partially attained. Loschmidt in 
"Vienna had shown, and not much later Stoney inde¬ 
pendently in England showed, how to deduce from 
Cllausius and Maxwell’s kinetic theory of gases a superior 
limit to the number of atoms in a given measurable 
space. I was unfortunately quite unaware of what 
Loschmidt and Stoney had done when I made a similar 
estimate on the same foundation, and communicated it to 
Nature in an article on “The Size of Atoms.’’ But 
questions of personal priority, however interesting they 
may be to the persons concerned, sink into insignifi¬ 
cance in the prospect of any gain of deeper insight into 
the secrets of nature. The triple coincidence of inde¬ 
pendent reasoning in this case is valuable as confirma¬ 
tion of a conclusion violently contravening ideas and 
■opinions which had been almost universally held re¬ 
garding the dimensions of the molecular structure of 
matter. Chemists and other naturalists had been in the 
habit of evading questions as to the hardness or indivi¬ 
sibility of atoms by virtually assuming them to be infi¬ 
nitely 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 attract¬ 
ing or repelling other such centres with forces depend¬ 
ing 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 agree with those 
who have attributed to the atom occupation of space 
with infinite hardness and strength (incredible in any 
finite body); but we must realize it as a piece of matter 
of measurable dimensions, with shape, motion, and laws 
of action, intelligible subjects of scientific investiga¬ 
tion. 
The prismatic analysis of light discovered by Newton 
was estimated by himself as being “the oddest, if not 
the most considerable, detection which hath hitherto 
been made in the operations 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 nineteenth century. Our 
fundamental knowledge of the dark lines is due solely to 
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 instrument for chemical qualitative analysis 
in the hands of Fox Talbot and Herschcl, who first 
showed how, through it, the old “blowpipe 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 is judged by the unaided eye. But the 
application of this test to solar and stellar chemistry had 
never, I believe, been suggested, 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 be¬ 
tween his double dark line 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 coinci¬ 
dence by Professor W. H. Miller, which showed it to be 
.accurate to an astonishing 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 identical qualities which constitute that double 
bright line. 
(4) Observations made by Stokes himself, 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 w T hen 
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 
“ a 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 ap¬ 
proximately circular elastic disk, it has two fundamental 
notes or vibrations of approximately equal pitch; and 
that tire periods of these vibrations are precisely the 
periods of the two slightly different yellow lights consti¬ 
tuting the double bright line D. 
(3) That when vapour of sodium is at a high enough 
temperature to become itself a source of light, each atom 
executes these two fundamental vibrations simultane¬ 
ously ; 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, according to a well-known general principle 
of dynamics, are set to vibrate in either or both of those 
fundamental modes, if some of the incident light is of one 
or other of their periods, or some of one and some of the 
other ; so that the energy of the waves of those parti¬ 
cular qualities of light is converted into thermal vibra¬ 
tions of the medium and dispersed in all directions, while 
light of all 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. 
(0) That other vapours than sodium are to be found in 
the atmospheres of the sun and stars by searching for 
substances producing in the spectra of artificial flames 
bright lines coinciding with other dark lines of the solar 
and stellar spectra than the Fraunhofer lino 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 had been shown to me by M. 
Duboscque Soleil, and the Abbe Moigno, in Paris in the 
month of October, 1850. A prism and lenses were ar¬ 
ranged 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 a cup. When pieces of copper and pieces of zinc 
were separately thrown into the cup, the spectrum ex¬ 
hibited, in perfectly definite positions, magnificent 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 
i used separately. 
To Ivirehhoff belongs, I believe, solely the great credit 
of having first actually sought for and found other metals 
than sodium in the sun by the method of spectrum ana¬ 
lysis. His publication of October, 1859, inaugurated the 
practice of solar and stellar chemistry, and gave spec- 
