THE CHEMICAL UNITY OF THE COSMOS
Uniformity in the chemical composition of the various classes of cosmical bodies was to a certain extent implied in the extended form of the nebular hypothesis formulated by Kant and Laplace, but there was no direct evidence of any relation between terrestrial and extra-terrestrial matter — other than that afforded by meteorites — until 1859. In that year the celebrated researches of Kirchhoff and Bunsen furnished the key to the interpretation of the spectra of celestial bodies, and gave birth to the new science of Astrophysics, in which we have witnessed such remarkable developments in recent years.
One of the first fruits of the union of Astronomy and Physics Avas the discovery that some of our familiar chemical elements exist also in the atmosphere of the sun, and in due course it was discovered that the same was true of the stars and other cosmical bodies. It was further found that the stars having spectra of different types could be arranged in a sequence which strongly suggested that the differences, for the most part, were not due to original differences of composition, but to the different stages which they had reached in an orderly development from masses of identical composition. The probability that nebulae were the parent masses out of which stars have been condensed — foreshadowed by Herschel’s observations on the apparent continuity of nebular and stellar forms — was also greatly strengthened when the spectroscope of Huggins revealed that the chief radiation from many of
these bodies was emitted by luminous gases. Hence, the con-
