1881.] on (he Tcachingii of Modeni Spccfroftcopy. 503 



The cliaiijTjos of spectra due to niolocnlar combinations and rc- 

 niTangoments have in addition to tlicir theorotit-al inii>( rtancc a great 

 l^raetieal interest, for they will aliord us some day a means of answer- 

 ing approximately a great many questions relating to the temperature 

 of sun and stars. The gases and vapours in tlie solar atmosphere 

 are for the greater part in the molecular condition in which they 

 give a line-spectrum, and we know of stars the spectra of which 

 resemble our solar spectrum very nearly. We shall not be far wrong 

 in ascribing to such stars a temperature similar to that of our sun. 

 Other stars have absorbing envelopes showing spectra of fluted bands. 

 We know that fluted bands belong to a more com2)lex molecular 

 condition, which only can exist at lower temperatures. These stars, 

 therefore, must have a lower temperature than our sun. Dr. Huggins, 

 who has succeeded in obtaining most valuable photographs of star- 

 spectra, has been able to classify and arrange star-spectra; and it is 

 more than likely that in the series of stars arranged in order by 

 means of their spectra, we have at one end those of the highest, at 

 the other those of the lowest temperature. We are as yet far from 

 being able to assign any particular temperature to a star, but the 

 question by means of the spectroscope has been reduced to one which 

 can be decided in our laboratories, and however difficult it may be, we 

 may rest assured that it will ultimately be solved. As to our sun, 

 its temperature has been the subject of many investigations. Attempts 

 have been made to deduce it (at least approximately) from the amount 

 of heat it sends out. Different experimental laws have been proposed 

 to connect together the heat radiation of a body, and the temperature 

 of that body. The first law which was thus proposed gives ten million 

 degrees Centigrade as a lower limit ; the second law reduces that 

 lower limit to a little over 1500 degrees. Both these laws we now 

 know to be wrong. More accurate laws give something like ten or 

 twenty thousand degrees, but the whole method employed is one which 

 is open to a great many objections. 



We measure the combined heat radiation of different layers on the 

 solar surfaces, all of which are at different temperatures, and we 

 observe only an average effect which is much influenced by the 

 absorption in the outer layers of the solar atniosi)here and in the 

 corona, and does not admit of easy interpretation. The spectroscopic 

 method, which is yet in its infancy, has the advantage that we can 

 observe separately each layer of the sun, and we thus examine the 

 temperature not as an average, but for every jiart of the solar body. 

 Our way to proceed would consist in carefully observing the spectra 

 in different layers of the sun. Supposing we observe a change at one 

 point, we may investigate at what temperature that change takes 

 place, and we may then ascribe; the same temperature to that particular 

 place at the solar surface, if no other cause has interfered which may 

 have affected our result. This last conditional limitation leads us to 

 the discussion of the important but difficult question, whether we can 

 determine any such interfering cause, which, not being temperature, 



