XVIII.] REPLIES TO SPECIAL OBJECTIONS. 151 



this layer. He is dealing with temperatures which are so much 

 greater than the temperatures with which we work in the laboratory, 

 that such assumptions must be regarded as quite arbitrary. 



" Mr. Homer Lane, in his classical paper on the theoretical tem- 

 perature of the sun,* makes the assumption that Dulong and Petit 's 

 law of radiation is true for solar radiation, and he uses it to calculate 

 the temperature of the radiating layer, which he finds to be 28,000 F. 

 That is, he uses an empirical law, obeyed possibly at laboratory tem- 

 peratures in radiation from hot solids, to express the radiation at 

 enormous temperatures from a hot layer of gas which has layers of 

 gas of all sorts of temperatures above and below it. 



"It seems to me that we know too little about the phenomenon of 

 radiation from layers of gas with denser and hotter layers below and 

 rarer and colder layers above to allow of any weight being placed upon 

 these assumptions of Bitter or Homer Lane. In a star we have layers 

 of fluid at all sorts of temperature and density. We have no labora- 

 tory knowledge of radiation that is applicable. We know very little 

 about any star except our own sun. *.**,-** Assumptions like 

 those of Homer Lane and Ritter may lead to results which are 

 altogether wrong." 



Finally, I may refer to two more objections from another quarter, 

 the first relates to the connection which I have insisted upon between 

 the length of the continuous spectrum and the temperature of the light 

 source, and I have stated that this is based upon KirchhofF s law. To 

 this it is objected that rays far up in the ultra-violet can be emitted 

 from bodies not at a high temperature. The inference is that the stars 

 with the longest spectra may be cold. But they are connected with 

 the sun by an unbroken chain of sequences in the phenomena. Then 

 is the sun also cold 1 



Again, it is urged that the phenomena of the gaseous stars instead 

 of being due to high temperature, are caused by phosphorescence. 

 Where then are Crookes's phosphorescent spectra 1 If this objection 

 implies that hydrogen can be made to phosphoresce so as to give us 

 Pickering's spectrum, the objector should have made the experiment 

 before he committed himself to such an objection.. 



* American Journal of Science anl Arts, 2nd series, vol. i, p. 57, 1870. 



