IN CONNEXION WITH THE SPECTEUM OF THE SUN. 
651 
Secondly. I have convinced myself that this is the spectrum of the oxide, because in 
air , after the first application of heat, the spectra and metallic lines are in the main the 
same, while in hydrogen the spectra are different for each compound, and true metallic 
lines are represented according to the volatility of the compound, only the very longest 
lines being visible in the spectrum of the least volatile compound. 
In proof of this statement I append a drawing (Plate LII.), on the scale of the maps of 
the last paper, representing the spectra of the chloride, bromide, and iodide of strontium. 
In order to avoid the introduction of the oxide spectrum, and so to secure the differentia- 
tion of the three spectra if possible, they were observed in hydrogen, which gas had been 
carefully treated in such a manner as to secure as far as possible the exclusion of any 
trace of oxygen. It will be seen at a glance that the spectra differ not only from the 
spectrum given by the metal, or by its salts in air at a high temperature, but considerably 
amongst themselves. In the experiments care was taken to keep the spark temperature 
as low as possible; and of course no jar was used, its presence in the circuit being 
instantly felt by the introduction of large numbers of metallic lines and the disappearance 
of the banded spectrum. 
The drawings were made from readings of the scale of the Steinheil spectroscope, and 
then approximately laid down on a scale of wave-lengths. 
General Statement. 
These and other facts which I have observed can be included in a general statement 
such as the following. 
1. A compound body, such as we have been considering, has as definite a spectrum as 
a simple one ; but while the spectrum of a simple metal consists of lines, the number 
and thickness of some of which increase with molecular approach, the spectrum of the 
compound consists in the main of channelled 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 their approach or recess, 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 channelled lines in the spectrum of the compound ; 
and in each case the greatest simplicity of the spectrum depends upon the greatest sepa- 
ration of molecules, and the greatest complexity (a continuous spectrum) upon their 
nearest approach. 
2. The heat required to act upon a compound, 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 dissociation; and as the metal lines increase in 
number, the compound bands thin out*. 
* The above statement is confirmed by the following experiments. A bead of strontic chloride was inter- 
posed between two aluminium electrodes; the induced current, without ajar, was then passed. The red-band 
spectrum of the oxide was very intense, and the only metallic line of any strength was 4607'5. The wire and 
bead soon became red-hot, and the latter evaporated, the spectrum disappearing. Ajar of 186 square centims. 
