58 ABSORPTION AND EMISSION. 



at a white heat, all the rays of the visible spectrum are 

 emitted. The power of emitting any definite ray depends, 

 consequently, on the temperature of the heated body. It 

 does not follow, from this, however, that the emissive powers 

 of all bodies are equal ; for bodies differ greatly from one 

 another in their capacity of receiving heat. 



(74) "When the light emitted from an incandescent solid 

 or liquid body is analyzed by the prism, it is found to contain 

 all the colours of the solar spectrum, although in varying 

 proportions. Its spectrum is accordingly continuous. The 

 case is altogether different when the incandescent body is a 

 vapour, or a gas. In this case the spectrum is found to corr- 

 sist of a small number of definite rays, separated from one 

 another by broad, dark intervals ; and the band of light is 

 discontinuous. 



(75) There are different modes of exhibiting these phe- 

 nomena, according to the degree of heat required for incan- 

 descence. For some substances the heat of a spirit-lamp is 

 sufficient ; while for others the heat of the oxy-hydrogen 

 light, or even that produced by the electric spark, is neces- 

 sary. The Bunsen lamp, which itself emits a very feeble 

 light, affords a very convenient mode of exhibiting the spectra 

 of the former class. When a small piece of common salt 

 (chloride of sodium) is introduced into this flame, the light 

 emitted is a bright orange-yellow ray, which is of the same 

 refrangibility as the ray D of the solar spectrum. And the 

 case is the same with all the salts of the same metal, the 

 light of the metallic vapour overpowering that of the other 

 elements of the compound. By such means Bunsen and 

 Eoscoe studied the lights of the alkaline metals, sodium, 

 potassium, lithium, calcium, strontium, and barium ; and they 

 found that the same process was applicable to the vapours of 



