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Kirchhoff, in so far as it supposed cloud to be the cause of darkness; but differed in so far as it assigned the cause of such cloud. More pressing matters prevented me from developing the idea for some time; and, afterwards, I was deterred from including it in the revised edition of the essay, by its inconsistency with the "willow-leaf" doctrine, at that time dominant.
At the same time, the Marquis de Laplace was working on a new theory of creation, which made the earth a little blotch in the nebulous sea out of which the planetary system had been formed and Bunsen and Kirchhoff, by the use of the spectroscope, were investigating the chemical composition of the stars and of our good neighbour, the sun, whose curious spots had first been noticed by Galileo.
Von Kirchhoff, in a brief impromptu speech, paid a high tribute to the German troops which were holding the French and said the decoration was not his but his troops'. And in a broken voice he remarked that these soldiers were sacrificing their lives for the Fatherland, but were called "Huns and Barbarians" for doing it.
He succeeds in showing that, agreeably to the law of Kirchhoff, the relation between the emitting and the absorbing power must be independent of the special properties of the body under observation, and he thus again meets with the laws of Planck and of Wien: unfortunately the calculation can only be made in the case of great wave-lengths, and grave difficulties exist.
In all the ordinary experiments of spectrum analysis the laws of Kirchhoff cannot therefore be considered as established, and yet the relation between emission and absorption is generally tolerably well verified.
All this seems wonderful enough, but even greater things were in store. In 1859 the spectroscope came upon the scene, perfected by Kirchhoff and Bunsen, along lines pointed out by Fraunhofer almost half a century before.
It was suspected by Stokes and by Fox Talbot in England, but first brought to demonstration by Kirchhoff and Bunsen, that these lines, which were known to occupy definite positions in the spectrum, are really indicative of particular elementary substances.
It is just to add, that Kirchhoff was unacquainted, when he undertook his investigation, either with the experiment of Foucault or the speculation of Stokes. For C. J. Ångström, on the other hand, perhaps somewhat too much has been claimed in the way of anticipation. His Optical Researches appeared at Upsala in 1853, and in their English garb two years later.
Kirchhoff knew that this was an action not peculiar to the sodium flame, and he immediately extended his generalisation to all coloured flames which yield sharply defined bright bands in their spectra.
In an investigation described by Kirchhoff as "classical," Swan had shown that 1/2,500,000 of a grain of sodium in a Bunsen's flame could be detected by its spectrum. He also proved the constancy of the bright lines in the spectra of hydrocarbon flames.
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