By 1811 he had greatly increased his observations of the nebulæ and could arrange them in series differing in extent, condensation, brightness, general form, possession of nuclei, situation, and in resemblance to comets and to stars. They ranged from a faint trace of extensive diffuse nebulosity to a nebulous star with a mere vestige of cloudiness. Herschel was able to make the series so complete that the difference between the members was no more than could be found in a series of pictures of the human figure taken from the birth of a child till he comes to be a man in his prime. The difference between the diffuse nebulous matter and the star is so striking that the idea of conversion from one to the other would hardly occur to any one without evidence of the intermediate steps. It is highly probable that each successive state is the result of the action of gravity. In his last statement, 1818, he admitted that to his telescopes the Milky Way had proved fathomless, but on "either side of this assemblage of stars, presumably in ceaseless motion round their common center of gravity, Herschel discovered a canopy of discrete nebulous masses, such as those from the condensation of which he supposed the whole stellar universe to be formed." In the theory of the evolution of the heavenly bodies, as set forth by Kant, Laplace, and Herschel, it was assumed that the elements that composed the earth are also to be found elsewhere throughout the solar system and the universe. The validity of this assumption was finally established by spectrum analysis. But this vindication was in part anticipated, at the beginning of the nineteenth century, by the analysis of meteorites. In these were found large quantities of iron, considerable percentages of nickel, as well as cobalt, copper, silicon, phosphorus, carbon, magnesium, zinc, and manganese. REFERENCES G. F. Becker, Kant as a Natural Philosopher, _American Journal of Science_, vol. V (1898), pp. 97-112. W. W. Bryant, _A History of Astronomy_. Agnes M. Clerke, _History of Astronomy during the Nineteenth Century_. Agnes M. Clerke, _The Herschels and Modern Astronomy_. Sir William Herschel, Papers on the Construction of the Heavens (_Philosophical Transactions_, 1784, 1811, etc.). A. R. Hinks, _Astronomy_ (Home University Library). E. W. Maunders, _The Science of the Stars_ (The People's Books). CHAPTER XII THE REIGN OF LAW--DALTON, JOULE