264- M. Melloni on the Radiations of Incandescent Bodies^ 



under trial became simultaneously luminous as the temperature 

 was rising, and also simultaneously lost this property when, 

 after being removed from the fire and carried into a dark place, 

 the temperature fell by radiation. 



It is important to observe, that in some cases, for example 

 with lead, the matter contained in the tube became incandes- 

 cent some time after it had melted : this shows that the tem- 

 perature necessary for the first manifestation of the luminous 

 condition does not depend on the solid state of the body sub- 

 mitted to the experiment, and that it takes place in all sub- 

 stances which maintain themselves in the liquid condition 

 without boiling at 977' F. Professor Draper excepts from 

 this law the fluoride of calcium and carbonate of lime, which 

 become luminous much sooner. These cases do not in reality 

 belong to the phsenomena we are now considering, but to those 

 o{ phosphorescence^ which require a lower degree of heat, ac- 

 cording to the nature of the substance. I may add that, re- 

 garding the question in a general point of view, we should 

 also except those cases in which the development of light 

 arises in chemical combinations. 



But these two exceptions are easily recognized by the quality 

 of the first colours that appear. Thus the carbonate of lime 

 emits, the moment it becomes visible, a white light, and the 

 fluoride of calcium a blue light. Sulphur appears yellow when 

 it combines chemically with copper, and blue when it unites 

 with oxygen. Some philosophers of the highest eminence, 

 among others M. Biot, suppose that the first light disengaged 

 by incandescent bodies is of the latter colour; and they have 

 accounted for this on the principles of a theory which is now 

 almost universally abandoned *. We shall presently see what 

 is the probable origin of the various tints that arise from phos- 

 phorescent bodies and chemical combinations : we shall merely 

 observe here, that the colour mentioned by Biot occurs in 

 flames ; and that these cases of combustion, belonging to the 

 class of chemical combinations, ought to be carefully distin- 

 guished from incandescence, properly speaking, which arises 

 directly and solely, as we have already said, from an elevation 

 of temperature in the body, and which always commences 

 with a red light. 



As to the exact degree of this temperature, the objections 

 which might be raised against the mode employed by our 

 author are of very little importance ; if we compare the results 

 at which he arrives with those that have been obtained by 

 Wedgewood and Daniell, the difference is only 30^ in excess 

 for the first case, and 3° too little in the second. The differ- 

 ences are much greater when compared with the deductions 

 • Biot, Traitede Thynque, vol. iv. p. 617. 



