4 INORGANIC EVOLUTION. [CHAP. 



without any breaks or gaps. We have what is called a continuous 



The Continuous Spectrum. 



If we burn a piece of paper, or a match, or ordinary coal-gas, we get 

 a white light identical to that given us by the candle ; solids which do 

 not liquefy when made white-hot, and liquids which do not volatilise under 

 the same condition, and some dense gases when heated, do the same. 



This effect is produced because there is light of every wave-length 

 to produce an image of the needle (or the slit) ; these images blend 

 together continuously from one end of the spectrum to the other. 



Let us then consider this fact established, namely, that solid or 

 liquid bodies and dense gases, when heated to a vivid incandescence, 

 give a continuous spectrum. Under these circumstances the light to 

 the eye, without the spectroscope, will be white, like that of a candle or 

 white-hot poker. 



The Length of the Continuous Spectrum -varies with Temperature. 



If we put a poker in a fire, it becomes red-hot ; if we heat a platinum 

 wire by passing a feeble current of electricity along it, it becomes red- 

 hot like the poker. 



In both cases examination by means of the prism shows that the 

 red end only of the spectrum is visible. But if the poker or wire be 

 gradually heated more strongly, the yellow, green, and blue rays will 

 successively appear. Finally, when a brilliant white heat has been 

 attained, the whole of the colours of the spectrum will be present. 



Hence we learn that *if the degree of incandescence be not high, the 

 light will only be red. But, so far as the spectrum goes and it will 

 expand towards the violet ! as the 'incandescence increases, as before 

 stated it will be continuous. ' >% 



The red condition comes from the absence of blue light; the white 

 condition comes from the gradual addition of blue as the temperature 

 increases. 



One of the laws formulated by Kirchhoff in the infancy of spectro- 

 scopic inquiry has to do with the kind of radiation given out by bodies 

 at different temperatures. The law affirms that the hotter a< mass of 

 matter is the further its spectrum extends into the ultra-violet. 



. Gaslight is redder than the light of an incandescent lamp because 

 the latter is hotter. The carbons in a so-called arc-lamp give out a 

 bluish- white light because they are hotter still. 



By similar reasoning from experiment we are bound to consider 



