48 CORRELATION OF PHYSICAL FORCES. 



light which would appear as a bright line or lines in the spec- 

 trum produced by its own light. 



By comparing the positions of the bright lines in the 

 spectra of various metals with those of the dark lines in the 

 solar spectrum, several of them are found to be in identically 

 the same place : hence it is inferred, and the inference seems 

 reasonable, that the metals which show luminous lines in their 

 spectra, identical in position with dark lines in the solar spec- 

 trum, exist in the sun, and are diffused in a gaseous state in 

 its atmosphere. It does not seem to me necessary to this 

 conclusion to assume that the sun is a solid mass of incandes- 

 cent matter : it may well be that what we term the photo- 

 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 in an incandescent temperature ; indeed, the pro- 

 tuberances and red light seen at the period of total eclipses 

 afford some evidence of an atmosphere exterior to the photo- 

 sphere. It would, however, be out of place here to speculate 

 on these subjects : the point which concerns us is the analogy 

 of heat and light, which these discoveries illustrate. Kirch- 

 hoff 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 theory 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 produc- 

 ing 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, 



