24 PLEASANT WAYS IN SCIENCE. 



to believe that an ignited* gas in the solar atmosphere 

 should not be indicated by dark lines in the solar spectrum, 

 and should appear not to act under the law, 'a gas when 

 ignited absorbs rays of the same refrangibility as those it 

 emits.' But, in fact, the substances hitherto investigated 

 in the sun are really metallic vapours, hydrogen probably 

 coming under that rule. The non-metals obviously may 

 behave differently. It is easy to speculate on the causes of 

 such behaviour ; and it may be suggested that the reason of 

 the non-appearance of a dark line may be that the intensity 

 of the light from a great thickness of ignited oxygen over- 

 powers the effect of the photosphere, just as, if a person 

 were to look at a candle-flame through a yard thickness of 

 sodium vapour, he would only see bright sodium lines, and 

 no dark absorption." 



The reasoning here is not altogether satisfactory (or 

 else is not quite correctly expressed). In the first place, 

 the difficulty dealt with has no real existence The law 

 that a gas when glowing absorbs rays of the same refran- 

 gibility as it emits, does not imply that a gas between a 

 source of light and the observer will show its presence by 

 spectroscopic dark lines. A gas so placed does receive from 

 the source of light rays corresponding to those which it 

 emits itself, if it is cooler than the source of light ; and it 

 absorbs them, being in fact heated by means of them, though 

 the gain of temperature may be dissipated as fast as received ; 

 but if the gas is hotter, it emits more of those rays than it 

 absorbs, and will make its presence known by its bright lines. 

 This is not a matter of speculation, but of experiment On 



The word "ignited" may mislead, and indeed is not correctly 

 used here. The oxygen in the solar atmosphere, like the hydrogen, is 

 simply glowing with intensity of heat No process of combustion is 

 taking place. Ignition, strictly speaking, means the initiation of the 

 process of combustion, and a substance can only be said to be ignited 

 when it has been set burning. The word glowing is preferable ; or if 

 reference is made to heat and light combined, then "glowing with in- 

 tensity of heat " seems the description most likely to be correctly under- 

 stood. 



