M. Angström has added aluminium and manganese to the seven metals whose vapours M. Kirchhoff has shown to exist in the atmosphere of the sun, but he thinks it doubtful whether barium, zinc, or copper are solar metals, for although their brighter lines correspond with distinct dark solar lines, their weaker lines do not. Strontium is doubtful also, for one of its strongest bright lines is not coincident with any dark line. Though both iron and nickel are decidedly solar metals, yet as cobalt is doubtful, it cannot be presumed that meteorites are of solar origin.
The spectrum of luminous magnesium has many green lines perfectly coincident with those in the solar spectrum, so there is no doubt of that metal being a constituent of the sun’s atmosphere. But there are magnesium rays as well as some of iron of such high refrangibility that in Mr. Stokes’s long spectrum they are situated ten times as far from H as the whole length of the visible spectrum from A to H. These highly refrangible rays only become visible at the exalted temperature of the electric spark, and as they are not found in the solar spectrum, it is inferred that the heat of the sun is inferior to that of the electric spark.[[21]] Mr. Roscoe observes that this conclusion would only be legitimate if we knew that these rays of high refrangibility are not absorbed in passing through the atmosphere.
These are some of the most striking results of the numerous investigations that have been made since M. Kirchhoff published his discoveries, for the subject is anything but exhausted.
The intensely vivid light of a magnesium flame is rich in violet and extra-violet rays, partly due to the incandescent vapour of magnesium, and partly to the intensely heated magnesia formed by the combustion. The properties of this light having been examined and compared with those of the sun by Professors Roscoe and Bunsen, with a view to photographic purposes, they came to the conclusion that ‘the steady and equable light evolved by magnesium wire, burning in the air, and the immense chemical action thus produced, render this source of light valuable as a simple means of obtaining a given amount of chemical illumination, and that the combustion of this metal constitutes a definite and simple source of light for the purpose of photochemical measurement.’
Bright lines of two different metals sometimes coincide with the same black line, that is, they appear to have the same reverse as an iron and a magnesian line, an iron and a nickel line, and some others; but it is not known whether the coincidence be real or apparent.
M. Kirchhoff has proved that neither gold, silver, tin, lead, antimony, arsenic, mercury, lithium, cadmium, and some others are constituents of the sun, because none of their bright lines are coincident with any of the dark lines of the solar spectrum. This negative discovery does no less honour to M. Kirchhoff than the proof of so many substances being common to the earth and sun.
Since all incandescent solid and liquid bodies give a continuous spectrum which exhibits no dark lines, M. Kirchhoff conceives that the sun consists of a solid or liquid nucleus, heated to the temperature of the most dazzling whiteness, and that it is surrounded by a luminous gaseous atmosphere of somewhat lower temperature, endowed with the law of exchanges. The spectra of Arcturus, Capella, and many other fixed stars are crossed by dark lines similar to, and often coincident with, the dark lines in the solar spectrum; therefore, it may be concluded that their structure is to a certain extent the same with that of the sun.
Numerous observations have been made on the spectra of the fixed stars, both in Britain and on the Continent. In England, Mr. Huggins and Professor W. A. Miller have published tables of the measures of about ninety dark lines in the spectrum of Aldebaran, nearly eighty in that of α Orionis or Betelgeux, and fifteen in that of β Pegasi, with diagrams of the two first which include the results of a comparison of the spectra of various terrestrial elements with those of the stars. Thus coloured lines of sodium, magnesium, calcium, hydrogen, iron, bismuth, tellurium, antimony and mercury were found to be coincident with some of the dark lines in the spectrum of Aldebaran, and besides these there are numerous lines in the spectrum of this star which are probably due to forms of matter unknown to us. Coloured lines of sodium, magnesium, calcium, iron and bismuth, coincided with dark lines in the spectrum of α Orionis; and β Pegasi had a spectrum closely resembling that of α Orionis, but much fainter.
Between forty and fifty stars were examined, and it was observed that the solar lines C and F corresponding to hydrogen, which are present in the spectra of nearly all the stars, are wanting in those of α Orionis and β Pegasi. With a few exceptions, the terrestrial elements hydrogen, sodium, magnesium, and iron, which appear to be most widely diffused through the stars, are precisely those which with the exception of magnesium are essential to life as it exists upon the earth. Besides, the elements hydrogen, sodium, and magnesium, represent the ocean, which is an essential part of a world similar to the earth. Should any planets revolve round α Orionis and β Pegasi, they probably would have no hydrogen, consequently, no ocean and no water: therefore, they could not be inhabited by beings constituted as we are.
Padre Secchi, the Roman astronomer, divides the stars into three types; the first and most dominant type includes Sirius, α Lyræ, and other white stars, which invariably contain hydrogen of high temperature, and are denoted by a black line in their spectra, which coincides with the solar line F; and there is another band also probably due to hydrogen in the violet half of the stars visible to the naked eye belonging to this group. A singular modification of this group, however, occurs in the stars of the constellation Orion, which so rarely show any deviation from one type, that, with the exception of α Orionis or Betelgeux, they may be said to form a family distinguished from all the other stars in the sky; their spectra are crossed by fine lines, faint in the violet, with a band more or less visible in F. γ Cassiopeiæ and β Lyræ differ from the stars of the first type in having a bright band near the solar line F, instead of a black one.[[22]]