6o8 THE POPULAR SCIENCE MONTHLY. 



These results enable us to make the following statement : 



A compound body, such as a salt of calcium, has as definite a spec- 

 trum as that given by the so-called elements ; but while the spectrum 

 of the metallic element itself consists of lines, the number and thick- 

 ness of some of which increase with increased quantity, the spectrum 

 of the compound consists in the main of channeled spaces and bands, 

 which increase in like manner. 



In short, the molecules of a simple body and a compound one are 

 affected in the same manner by quantity in so far as their spectra are 

 concerned ; in other words, both spectra have their long and short 

 lilies, the lines in the spectrum of the element being represented by 

 bands or fluted lines in the spectrum of the compound ; and in each 

 case the greatest simplicity of the spectrum depends upon the smallest 

 quantity, and the greatest complexity upon the greatest. 



The heat required to act upon such a compound as a salt of calcium, 

 so as to render its spectrum visible, dissociates the compound according 

 to its volatility ; the number of true metallic lines which thus appear is 

 a measure of the quantity of the metal resulting from the dissociation, 

 and as the metal lines increase in number, the compound bands thin out. 



These results bring us face to face with the subject-matter of the 

 recent work. 



First with regard to impurity elimination. I find that, although the 

 method is good for detecting and eliminating impurities, there are still 

 short-line coincidences between metals which are pure. 



This " higrher law " has come out in the followinof manner : 



For the last four years I have been engaged upon the preparation 

 of a map of the solar spectrum on a large scale, the work including a 

 comparison of the Fraunhofer lines with those visible in the spectrum 

 of the vapor of each of the metallic elements in the electric arc. 



To give an idea of the thoroughness of the work, at all events in 

 intention, I may state that the complete spectrum of the sun, on the 

 scale of the working map, will be half a furlong long ; that to map the 

 metallic lines and purify the spectra in the manner described, more 

 than 100,000 observations have been made and about 2,000 photographs 

 taken. 



In some of these photographs we have vapors compared with the 

 sun ; in others vapors compared with each other ; and others again 

 have been taken to show which lines are long and which are short in 

 the spectra, 



A rigorous application of the system of impurity elimination formed, 

 of course, a large part of the work. 



The final reduction of the photographs of all the metallic elements 

 in the region 39-40 — a reduction I began in the early part of last year 

 — summarized all the observations of metallic spectra compared with 

 the Fraunhofer lines accumulated during the whole period of observa- 

 tion, and all the results of the impurity elimination. 



