30 



Mr. J. N.' Lockyer, 



[Dec. 18, 



flame at wave-length. 4570*3, a difference of more than sixteen divisions 

 of the scale. 



I am now preparing maps showing the phenomena observed at various 

 heat-levels. I think it is not too much to hope that a carefnl study of 

 such maps, showing the results already obtained, or to be obtained, 

 at varying temperatures, controlled by observations of the condi- 

 tions under which changes are brought about, will, if we accept the 

 idea that various dissociations of the molecules present in the solid 

 are brought about by different stages of heat, and then reverse the 

 process, enable us to determine the mode of evolution by which the 

 molecules vibrating in -the atmospheres of the hottest stars associate 

 into those of which the solid metal is composed. I put this suggestion 

 forward with the greater confidence, because I see that help can be 

 got from various converging lines of work. To some of these I 

 may briefly allude here : 



1. "We have the lines present in the solar spectrum, and absent 

 from it. 



Example. — The red potassium line present in the flame is absent 

 from the sun ; some of the other lines are present. 



2. We have the varying thicknesses of the lines of any one element 

 in the sun to compare with the thicknesses produced at different 

 temperatures in the laboratory. 



Example. — The various lines of magnesium, notably b, the most 

 refrangible line given by Thalen, and the other blue line. 



3. We have the remarkable behaviour of metals vaporised in a 

 vacuum at the lowest temperatures. 



Example. — Sodium gives us D, potassium gives us the triplet in the 

 green-yellow ; calcium gives us the line in the blue ; thus separating 

 those lines from all the others of those metals. 



4. We have the remarkable behaviour of the same vapours under 

 like circumstances, the temperature alone being changed ; when this 

 is increased lines visible under ordinary conditions are brought in, 

 and are seen in different parts of the tube, so that each line in turn 

 (and therefore, I presume, each molecule which produces it) is 

 separated from those with which it is generally seen in company. 



Example. — By increasing the temperature we get the green line of 

 sodium without D, and some of the magnesium lines have been seen 

 separated from the others. 



5. We have the power of determining the lower states by means of 

 absorption phenomena, and then of observing the radiation of the 

 vapours produced by the passage of a feeble current of electricity. 



Example. — The fluted spectrum of sodium described by Roscoe and 

 Schuster is instantly abolished by this means, and a brightening of the 

 green and a considerable thickening of the dark yellow lines is seen. 



