388 A Short History of Astroncmy [CH. xin. 



Robert Kirchhoff (1824-1887) of Heidelberg, who at first 

 worked in co-operation with the chemist Bunsen. 



Kirchhoff shewed that a luminous solid or liquid or, 

 as we now know, a highly compressed gas gives a con- 

 tinuous spectrum ; whereas a substance in the gaseous 

 state gives a spectrum consisting of bright lines (with or 

 without a faint continuous spectrum), and these bright 

 lines depend on the particular substance and are charac- 

 teristic of it. Consequently the presence of a particular 

 substance in the form of gas in a hot body can be inferred 

 from the presence of its characteristic lines in the spectrum 

 of the light. The dark lines in the solar spectrum were 

 explained by the fundamental principle often known as 

 KirchhofFs law that a body's capacity for stopping or 

 absorbing light of a particular wave-length is proportional 

 to its power, under like conditions, of giving out the 

 same light. If, in particular, light from a luminous solid 

 or liquid body, giving a continuous spectrum, passes through 

 a gas, the gas absorbs light of the same .wave-length as that 

 which it itself gives out : if the gas gives out more light 

 of these particular wave-lengths than it absorbs, then the 

 spectrum is crossed by the corresponding bright lines ; 

 but if it absorbs more than it gives out, then there is a 

 deficiency of light of these wave-lengths and the corre- 

 sponding parts of the spectrum appear dark that is, the 

 spectrum is crossed by dark lines in the same position as 

 the bright lines in the spectrum of the gas alone. Whether 

 the gas absorbs more or less than it gives out is essentially 

 a question of temperature, so that if light from a hot solid 

 or liquid passes through a gas at a higher temperature a 

 spectrum crossed by bright lines is the result, whereas if 

 the gas is cooler than the body behind it dark lines are 

 seen in the spectrum. 



300. The .presence of the Fraunhofer lines in the 

 spectrum of the sun, shews that sunlight comes from a 

 hot solid or liquid body (or from a highly compressed gas), 

 and that it has passed through cooler gases which have 

 absorbed light of the wave-lengths corresponding to the 

 dark lines. These gases must be either round the sun or 

 in our atmosphere ; and it is not difficult to shew that, 

 although some of the Fraunhofer lines are due to our 



