xin.] AN ANALOGY. 181 



known compound body is decomposed. With ordinary com- 

 pounds, such as chloride of calcium and so on, one can watch the 

 precise moment at which the compound is broken up when the 

 calcium begins to come out ; and we can then determine the 

 relative amount of dissociation by the number and brightness of 

 the lines of calcium which are produced. Similarly with regard 

 to these flutings we can take iodine vapour, which gives us 

 a fluted spectrum, and we can then increase the temperature 

 suddenly, in which case we no longer get the fluted spectrum at 

 all ; or we may increase it so gently that the tme lines of iodine 

 come out one by one in exactly the same way that the lines of 

 calcium become visible in the spectrum of the chloride of cal- 

 cium. We end by destroying the compound of calcium and its 

 spectrum in the one case, and by destroying the fluted spectrum 

 of iodine in the other, leaving, as the result in both cases, the 

 bright lines of the constituents in the one case calcium and 

 chlorine : in the other case iodine itself. 



I have by no means exhausted the list of difficulties which 

 were gradually presented to us when we considered that both 

 in the sun and in our laboratories spectrum analysis brought 

 before us phenomena due to the vibrations of unique, absolutely 

 similar " chemical atoms." Not only were there differences, but 

 the differences worked in different ways, whether we passed 

 from low to high temperatures in laboratory work, or from the 

 general spectrum to the prominence- or spot-spectrum in 

 the sun. 



But I have said enough for my present purpose ; details on 

 some points I have referred to must be gone into in the next- 

 chapter. 



