
Aug. 3, 1871] 

application to solar and stellar chemistry ; but because we might 
now be in possession of the inconceivable riches of astronomical 
results which we expect from the next ten years’ investigation by 
spectrum analysis, had Stokes given his theory to the world when 
it first occurred to him. 
To Kirchhoff belongs, I believe, solely the great credit of hav- 
ing first actually sought for and found other metals than sodium in 
the sun by the method of spectrum analysis. His publication of 
October 1859 inaugurated the practice of solar and stellar che- 
mistry, and gave spectrum analysis an impulse to which in a 
great measure is due its splendidly successful cultivation by the 
labours of many able investigators within the last ten years. 
To prodigious and wearing toil of Kirchhoff himself, and of 
Angstrom, we owe large-scale maps of the solar spectrum, in- 
comparably superior in minuteness and accuracy of delineation to 
anything ever attempted previously. These maps now consti- 
tute the standards of reference for all workers in the field. 
Pliicker and Hittorf opened ground in adyancing the physics of 
spectrum analysis, and made the important discovery of changes 
in the spectra of ignited gases produced by changes in the phy- 
sical condition of the gas. The scientific value of the meetings 
of the British Association is weil illustrated by the fact that it 
was through conversation with Pliicker at the Newcastle meeting 
that Lockyer was first led into the investigation of the effects of 
varied pressure on the quality of the light emitted by glowing 
gas which he and Frankland have prosecuted with such admir- 
able success. Scientific wealth tends to accumulation according 
to the law of compound interest. Every addition to knowledge 
of properties of matter supplies the naturalist with new instru- 
mental means for discovering and interpreting phenomena of 
nature, which in their turn afford foundations for fresh generalisa- 
tions, bringing gains of permanent value into the great storehouse 
of philosophy. Thus Frankland, led, from observing the want 
of brightness of a candle burning in a tent on the summit of 
Mont Blanc, to scrutinise Davy’s theory of flame, discovered that 
brightness without incandescent solid particles is given to a purely 
gaseous flame by augmented pressure, and that a dense ignited 
gas gives a spectrum comparable with that of the light from an 
incandescent solid or liquid. Lockyer joined him ; and the two 
found that every incandescent substance gives a continuous spec- 
trum—that an incandescent gas under varied pressure gives 
bright bars across the continuous spectrum, some of which, from 
the sharp, hard and fast lines observed where the gas is ina state 
of extreme attenuation, broaden out on each side into nebulous 
bands as the density is increased, and are ultimately lost in the 
continuous spectrum when the condensation is pushed on till the 
gas becomes a fluid no longer to be called gaseous. More re- 
cently they have examined the influence of temperature, and have 
obtained results which seemed to show that a highly attenuated 
gas, which at a high temperature gives several bright lines, gives 
a smaller and smaller number of lines, of sufficient brightness to 
be visible, when the temperature is lowered, the density being 
kept unchanged. I cannot refrain here from remarking how ad- 
mirably this beautiful investigation harmonises with Andrews’s 
great discovery of continuity between the gaseous and liquid 
states. Such things make the life-blood of science. Jn contem- 
plating them we feel as if led out from narrow waters of scholastic 
dogma to a refreshing excursion on the broad and deep ocean of 
truth, where we learn from the wonders we see that there are 
endlessly more and more glorious wonders still unseen. 
Stokes’s dynamical theory supplies the key to the philosophy of 
Frankland and Lockyer’s discovery. Any atom of gas when 
struck and left to itself vibrates with perfect purity its funda- 
mental note or notes. In a highly attenuated gas each atom is 
very rarely in collision with other atoms, and therefore is nearly 
at all times in a state of true vibration. Hence the spectrum of 
a highly attenuated gas consists of one or more perfectly sharp 
bright lines, with a scarcely perceptible continuous gradation of 
prismatic colour, In denser gas each atom is frequently in colli- 
sion, but still is for much more time free, in intervals between 
collisions, than engaged in collision ; so that not only is the atom 
itself thrown sensibly out of tune during a sensible proportion of 
its whole time, but the confused jangle of vibrations in every 
variety of period during the actual collision becomes more con- 
siderable in its influence, Hence bright lines in the spectrum 
broaden out somewhat, and the continuous spectrum becomes 
less faint. In still denser gas each atom may be almost as much 
time in collision as free, and the spectrum then consists of broad 
nebulous bands crossing a continuous spectrum of considerable 
brightness. When the medium is so dense that each atom is 
NATURE 

267 

always in collision, that is to say never free from influence of its 
neighbours, the spectrum will generally be continuous, and may 
present little or no appearance of bands, or even of maxima of 
brightness. In this condition the fluid can be no longer regarded 
as a gas, and we must judge of its relation to the vaporous or 
liquid states according to the critical conditions discovered by 
Andrews. 
While these great investigations of properties of matter were 
going on, naturalists were not idle with the newly recognised 
power of the spectroscope at their service. Chemists soon 
followed the example of Bunsen in discovering new metals in 
terrestrial matter by the old blow-pipe and prism test of Fox 
Talbot and Herschel. Biologists applied spectrum analysis to 
animal and vegetable chemistry, and to sanitary investigations. 
But it is in astronomy that spectroscopic research has been 
carried on with the greatest activity, and been most richly 
rewarded with results. The chemist and the astronomer have 
joined their forces. An astronomical observatory has now, 
appended to it, a stock of reagents such as hitherto was only to 
be found in the chemical laboratory. A devoted corps of volun- 
teers of all nations, whose motto might well be Uézgue, have 
directed their artillery to every region of the universe. The 
sun, the spots on his surface, the corona and the red and yellow 
prominences seen round him during total eclipses, the moon, the 
planets, comets, auroras, nebulxz, white stars, yellow stars, red 
stars, variable and temporary stars, each tested by the prism, 
was compelled to show its distinguishing prismatic colours, 
Rarely before in the history of science has enthusiastic perse- 
verance directed by penetrative genius produced within ten years 
so brilliant a succession of discoveries, It is not merely the 
chemistry of sun and stars, as first suggested, that is subjected to 
analysis by the spectroscope. Their whole laws of being are 
now subjects of direct investigation; and already we have 
glimpses of their evolutional history through the stupendous 
power of this most subtle and delicate test. We had only 
solar and stellar chemistry ; we now have solar and stellar 
physiology. 
It is an old idea that the colour of a star may be influenced by 
its motion relatively to the eye of the spectator, so as to be 
tinged with red if it moves from the earth, or blue if it moves 
towards the earth. William Allen Miller, Huggins, and Maxwell 
showed how, by aid of the spectroscope, this idea may be made 
the foundation of a method of measuring the relative velocity 
with which a star approaches to or recedes from the earth. The 
principle is, first to identify, if possible, one or more of the lines 
in the spectrum of the star, with a line or lines in the spectrum 
of sodium, or some other terrestrial substance, and then (by 
observing the star and the artificial light simultaneously by 
the same spectroscope) to find the difference, if any, between 
their refrangibilities. From this difference of refrangibility the 
ratio of the periods of the two lights is calculated, according to 
data determined by Fraunhofer from comparisons between the 
positions of the dark lines in the prismatic spectrum and in 
his own ‘‘interference spectrum ” (produced by substituting for 
the prism a fine grating). A first comparatively rough applica- 
tion of the test by Miller and Huggins to a large number of the 
principal stars of our skies, including Aldebaran, « Orionis, B 
Pegasi, Sirius, a Lyre, Capella, Arcturus, Pollux, Castor 
(which they had observed rather for the chemical purpose than 
for this), proved that not one of them had so great a veloclty as 315 
kilometres per second to or from the earth, which is a most 
momentous result in respect of cosmical dynamics. Afterwards 
Huggins made special observations of the velocity test, and suc« 
ceeded in making the measurement in one case, that of Sirius, 
which he then found to be receding from the earth at the rate of 
66 kilometres per second. This, corrected for the velocity of the 
earth at the time of the observation, gave a velocity of Sirius, 
relative to the Sun, amounting to 47 kilometres per second. The 
minuteness of the difference to be measured, and the smllness ofa 
the amount of light. even when the brightest star is observed, 
renders the observation extremely difficult. Still, with such 
great skill as Mr. Huggins has brought to bear on the investiga- 
tion, it can scarcely be doubted that velocities of many other 
stars may be measured. What is now wanted is, certainly not 
greater skill, perhaps not even more powerful instruments, but 
more instruments and more observers, Lockyer’s applications of 
the velocity test to the relative motions of different gases in the 
Sun’s photosphere, spots, chromosphere, and chromospheric 
prominences, and his observations of the varying spectra pre- 
sented by the same substance as it moyes from one position to 
