BUNSEN AND KIRCHHOFF 323 



internal constitution being quite insoluble. Bunsen and 

 Kirchhoff rendered this possible by taking up again the 

 optical investigations of Newton and Fraunhofer, which in 

 the previous forty-five years had hardly been advanced any 

 further; they started from the question of the emission and 

 absorption of light. 



It was known that hot bodies, at a temperature below 

 about 600° C, only emit the dark heat-rays, the infra-red 

 discovered by Scheele, and that as the temperature rises, 

 red light gradually appears (at a red heat), and finally the 

 whole visible spectrum (white heat). Solid and liquid 

 bodies exhibit in this respect, generally speaking, no great 

 difference; any piece of glowing charcoal allows all these 

 phenomena to be readily observed. Hot and glowing 

 gases or vapours however, such as we find in flame, behave 

 differently; at a sufficiently high temperature they are able 

 to glow with coloured instead of with white light, the colours 

 depending not so much upon the temperature, but rather 

 upon the material nature of the gas or vapour. This was 

 long known in pyrotechnics, and used for the production in 

 fireworks of flames of almost any desired colour, for which 

 purpose a large number of recipes always existed; it was 

 further also known that the flame of alcohol, which in itself 

 emits little light, becomes capable of emitting coloured light 

 when certain substances are brought into it. A yellow 

 colour in particular always readily appeared in the alcohol 

 flame, but often without any recognisable cause, so that 

 there was a certain doubt as to what the colour was due to. 

 What was wanted were clear experiments, which would 

 eliminate, with certainty, all matters of chance in order to 

 allow the true causes of the effects observed to be recog- 

 nised. When this had been attained in the matter of these 

 flame colourations, they might, for example, also be available 

 for indicating the presence of certain substances. 



Bunsen was the true investigator for such problems; he 



