210 On some Becent Spectroscopic Researclies. [April, 



the mode of its application to terrestrial objects and to tlie heavenly 

 bodies, are now too generally known to need description here. It 

 may be well, however, to state the principal forms nnder which all 

 spectra may be classed, and the interpretation which, in the present 

 state of our knowledge, we are justified in giving to these different 

 spectra, when the light has been emitted by bodies rendered 

 luminous by a high degree of heat. It is necessary to make this 

 distinction, as it is not purposed to describe the spectra of fluores- 

 cent and phosphorescent substances.* 



The spectra of all highly heated bodies may be referred to 

 three typical forms. 



First Form. — When the continuity of the coloured band into 

 which the light is dispersed by the prism remains unbroken eitber 

 by bright or by dark lines. Now, as a general rule, such a spec- 

 trum may be interpreted as telling us that the body which emitted 

 the light is in the solid or liquid state. Further than this, a con- 

 tinuous spectrum gives to us no information of the nature of the 

 source of the light, for all solid and liquid bodies are characterized 

 by a continuous spectrum, whatever their chemical nature may be. 



Such is the interpretation which, as a general rule, and unless 

 there should exist circumstances rendering a solid or hquid con- 

 dition of the source of light highly improbable, ought to be given 

 to a continuous spectrum. In certain cases gases may give a 

 spectrum which is continuous. Dr. Balfour Stewart has pointed 

 out that as gases and vapours possess a power of general or indis- 

 criminate absorption, in addition to the elective absorption peculiar 

 to each gas, it would follow that a gas when luminous would also 

 emit light of all reiVangibilities, producmg a continuous spectrum, 

 in addition to the spectrum of bright lines which is peculiar to the 

 gas, and further, that the intensity of this continuous spectrum 

 would be in proportion to the opacity of the gas. 



Besides this consideration, the researches of Plfickcr and Frank- 

 land have shown that, under certain conditions of temperature and 

 density, the bright hues of some gases expand, and so a spectrum 

 may be produced which would not be distinguishable from that of 

 the hght of a solid or hquid body. 



Second Form. — Spectra belonging to this class consist of bright 

 lines. These lines tell us that the source of the light is luminous 

 gas. Further, since, so far as observation extends, each gas and 

 vapour is distinguished by a set of lines peculiar to itself, it 

 becomes possible to discover if any of the substances known to 

 us are present in the source of light. This method of analysis is 

 not invalidated by the circumstance that the appearance of the lines 

 may be greatly modified, or even altogether changed, under dif- 



* For an account of the spectra of flTioro.'^cpnt and phosphorescent hodies, tlie 

 reader is referred to ' La Luniierc,' by Ya. Becquere), vol. i. Paris, 1867. 



