ON THE METAL INDIUM. 
336 
which, as they coincide with a dark solar line, reveal the presence of the particular metal 
in the sun's atmosphere. Kirchhoff’s maps now embrace the whole of the visible spec¬ 
trum from the line a in the extreme red, to the line g in the indigo ; beyond these 
limits the intensity of the light passing through his three prisms became too slight to 
enable him to draw the lines. The observations thus made of coincidences of metallic 
with solar lines in the red and indigo portions of the spectrum, confirm the conclusions 
drawn by Kirchhoff from his earlier observations, with the exception of the presence of 
potassium. This metal is not seen in the solar atmosphere; the potassium red line is 
not coincident with the solar line a, as it was supposed to be, nor with any other dark 
solar line. No metal, in addition to those previously observed, was found to possess 
lines coincident with solar lines, and hence the number of bodies known to be present in 
the sun has not been increased. 
The experiments of Mr. Huggins on the spectra of the metallic elements, made with 
an instrument of six prisms, although not yet published in full, promise to add greatly 
to our knowledge on this subject: one interesting observation may be cited ; viz. that 
the spectrum of sodium has been found to contain three pairs of lines in addition to 
those corresponding to the dark double line d, and that these also coincide with dark 
solar lines, adding to the evidence previously possessed of the -existence of sodium in 
the sun. The audience had been already made acquainted with Dr. Miller’s important 
researches on the photographic spectra of the metals, and with the valuable observations 
made by himself and Mr. Huggins on the spectra of the fixed stars. Connected with 
this part of the subject may be mentioned Professor Stokes’s interesting investigation on 
the long spectrum of the electric spark, in which he shows that the vapour of certain 
metals, such as iron and magnesium, when heated by the passage of an electric spark, 
emit rays of so high a degree of refraugibility, that they are situated at a distance from 
the lines h, ten times as great as that of the "whole visible spectrum from a to h. These 
highly refrangible rays only become visible at the highest temperatures, and they are not 
seen in the solar spectrum, although the less refrangible iron apd magnesium lines are 
present; hence it has been suggested that the temperature of the sun must be lower 
than that of the electric spark in which these lines are developed. This conclusion ap¬ 
pears legitimate only if we know that these rays of high refrangibility are not absorbed 
in passing through our atmosphere; and an investigation of great interest here presents 
itself for those who ascend into the higher regions of the atmosphere. 
The observations of Dr. Robinson upon metallic spectra have led this astronomer to 
doubt the validity of some of the conclusions arrived at by Ivirchholf concerning the 
existence of a separate and non-coincident set of lines in the spectrum of each metal. 
It seems, however, that Dr. Robinson employed only one prism and a low magnifying 
power, so that we must conclude that the observations from which he deduces the coin¬ 
cidence of certain lines as proving their identity in several metals, cannot impugn the 
results obtained by help of a larger instrument of sufficient power to resolve these appa¬ 
rent coincidences. 
The original statement made by Bunsen and Kirchhoff concerning the spectra of the 
metals still remains unopposed by a single well-established fact,—the statement, namely, 
that when a metal is heated up to a certain point, the spectrum of its incandescent 
vapour contains a number of fine bright lines which do not change their position with 
increase of temperature, and are not coincident with the lines of any other known 
substance. There is, however, no doubt of the fact that in the spectra of certain 
metals or metallic compounds new lines are developed by increase of temperature ; and 
also that certain metals, as calcium, barium, and strontium, yield spectra of two kinds ; 
one of these, seen at the lower temperature, and consisting of broad bands, being re¬ 
solved at a higher temperature into bright lines. These bright lines do not undergo any 
further change on elevation of temperature, and characterize the true metallic spectrum, 
whilst the band-spectrum is probably produced by the incandescent vapour of a metallic 
compound which is decomposed at a higher temperature. 
Our knowledge of the spectra of the non-metallic elements is, as yet, in a very in¬ 
complete state. To the researches of Pliicker we are especially indebted for information 
on this subject; he has shown that each metalloid possesses a peculiar and characteristic 
spectrum ; hydrogen, for instance, yielding only three bright lines, all of which are co¬ 
incident with dark solar lines ; and nitrogen exhibiting a complicated series of bands. 
Pliicker has lately come to the conclusion that many non-metallic elementary bodies, 
