xcviii REPORT — 1871. 



of prismatic colour. In. denser gas each atom is frequently in collision, but 

 still is for much more time free, iu intervals between collisions, than engaged 

 in collision ; so that not only is the atom itself thrown sensibly out of tuue 

 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 

 considerable 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 bauds crossing a continuous 

 spectrum of considerable brightness, "\^^hen the medium is so dense that 

 each atom is alwaj's in collision, that is to say never free from influence of 

 its neighbours, the spectrum wiU. generally be continuous, and may present 

 little or no ap])earance 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 Andi'cws. 



While these great investigations of properties of matter were going on, 

 naturalists were not idle with the newly recognized power of the spectro- 

 scope at their service. Chemists soon followed the example of Bunsen 

 in discovering new metals in terrestrial matter by the old blow-pipo 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 ob- 

 servatory 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 volunteers 

 of all nations, whose motto might well be ^ibique, have directed their artil- 

 lery 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, nebulae, white 

 stars, yellow stars, red stars, variable and temporary stars, each tested by the 

 prism was compelled to show its distinguishing colours. Earely before in 

 the history of science has enthusiastic perseverance directed by penetra- 

 tive genius produced within ten years so brilliant a succession of dis- 

 coveries. It is not merely the chemistry of sun and stars, as first sug- 

 gested, 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 stuj)endous 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 JIaxwell 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 refran- 

 gibilities. From this difference of refrangibility the ratio of the periods of 

 the two lights is calculated, according to data determined by Fraunhofer from 



