These results enable us to make the following statement:
A compound body, such as a salt of calcium, has as definite a spectrum as that given by the so-called elements; but while the spectrum of the metallic element itself consists of lines, the number and thickness 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 lines, 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 "higher law" has come out in the following 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 observation, and all the results of the impurity elimination.
