HEAT. 63 



sphere or luminous envelope of the sun has surrounding it a 

 more diffuse atmosphere containing vaporised metals, and 

 that the mass of the sun itself may be in a different state, 

 and not necessarily at an incandescent temperature ; indeed, 

 the protuberances and red light seen at the period of total 

 eclipses afford some evidence of an atmosphere exterior to 

 the photosphere. It would, however, be out of place here to 

 speculate on these subjects : the point which concerns us is 

 the analogies of heat and light, which these discoveries illus- 

 trate. Kirchoff has carried the analogy farther by showing 

 that a plate of tourmaline absorbs the polarised ray which 

 when heated it radiates. Thus, the phenomena of light are 

 imitated closely by those of radiant heat ; and the same the- 

 ory which is considered most plausibly to account for the 

 phenomena of the one, will necessarily be applied to the other 

 agent, and in each case molecular change is accompanied by 

 a change in the phenomenal effects. 



In certain cases heat appears to become partially con- 

 verted into light, by changing the matter affected by heat : 

 thus gas may be heated to a very high point without pro- 

 ducing light, or producing it to a very slight degree ; but the 

 introduction of solid matter for instance, the metal platinum 

 into the highly-heated gas instantly exhibits light. Whether 

 the heat is converted into light, or whether it is concentrated 

 and increased in intensity by the solid matter so as to become 

 visible, may be open to some doubt : the fact of solid matter, 

 when ignited by the oxyhydrogen jet decomposing water, as 

 will be presently explained, would seem to indicate that the 

 heat was rendered more intense by condensation in the solid 

 matter, as water is in this case decomposed by a heated body, 

 which body has itself been heated by the combining elements 

 of water. The apparent effect, however, of the introduction 

 of solid incombustible matter into heated gas, is a conversion 

 of heat into light. 



There is another method by which heat would probably 



