568 Radiation of Heat and Light from Heated Solid Bodies. 



proportions in which the radiation of longer period and 

 shorter period are present in the total radiation depends on 

 the radiating surface, other things being the same. In the 

 case of the sooted wire, the quantity of long-period radiation 

 is, in proportion, far in excess of that proceeding from a 

 bright metallic polished surface. Consequently, with the 

 same total electric energy supplied to both wires, the glass 

 tube containing the sooted wire becomes very much hotter 

 than the tube containing the bright wire. 



It has also been pointed out that with a substance like glass, 

 conducting badly and somewhat diathermanous, it is impos- 

 sible to tell how much heat is returned to the radiating wire 

 from the interior skin of the tube, which no doubt rises to a 

 high temperature during the experiment. To a certain ex- 

 tent, therefore, the results which we have recently obtained 

 must be considered as not strictly comparable with those 

 formerly obtained, in which a metallic envelope cooled with 

 water was used. 



The absolute value of the radiation observed ought to be 

 somewhat lower in amount than would have been found had 

 the enclosing envelope been of metal properly kept cool, and 

 the disturbance from this cause must have been relatively 

 greater in the case of the dull than in the case of the bright 

 wire. 



I cannot conclude without expressing my warmest thanks 

 to my assistant, Mr. Evans, whose aid has been invaluable. 

 Without the help which he has given me the experiments 

 could not have been carried out at all. The investi- 

 gation is still in progress, indeed, the present paper must 

 be regarded as a description of preliminary trials ; I hope 

 before long, however, to obtain further results in the same 

 direction of considerable interest. The temperatures shown 

 on the curves and in the tables have been obtained partly 

 by direct experiment and partly by using the results of 

 other experimenters who have given comparisons between 

 platinum resistances and temperatures. They must only 

 be regarded as approximations, and may possibly require 

 to be corrected. The amount of the correction cannot, how- 

 ever, alter the general conclusion that the polished surface 

 is much more economical for the production of light than the 

 sooted surface ; and that as far as our experiments have 

 gone, the polished platinum surface and the sooted surface 

 are practically at the same temperature w T hen they present a 

 similar appearance to the eye. 



