It will be observed, in reference to the theories we have just considered, that the process of evolution is supposed to take place from the smaller to the larger aggregates of matter. Beginning with an extreme condition of tenuity, by aggregation, the materials become successively larger and more complex. In passing backwards in the process we find the aggregates becoming less and less till they reach the “cosmical dust,” or “fire-mist,” out of which the primitive nebulæ were supposed to be formed. Receding still farther back, we have the universal atmosphere from which the fire-mist is assumed to have been derived. This universal atmosphere, though in a state of extreme tenuity, is, as we shall see, supposed by some to be in a more elemental form than anything revealed to us in the laboratory. The suggestion of the dissociation of the chemical elements and its application to stellar physics was, I think, first advanced by Sir Benjamin Brodie in 1866, and more fully in 1867. In the latter year views similar were considered more fully by Dr. T. Sterry Hunt. The question of the dissociation of elements has been ably discussed by Mr. Lockyer in his various writings. It has been suggested by Mr. Lockyer that the coincidence of rays emitted by different chemical elements when subjected to very high temperatures affords evidence of a common element in the composition of the metals producing the coincident rays. Mr. Lockyer states that many trains of thought suggested by solar and stellar physics point to the hypothesis that the _elements themselves, or at all events some of them, are compound bodies_.[67] This view was also put forward by Professor Graham, who says “that it is conceivable that the various kinds of matter now recognised in different elementary substances may possess one and the same element or atomic molecule existing in different conditions of mobility. The essential unity of matter,” he adds, “is an hypothesis in harmony with the equal action of gravity upon all bodies.” Similar views have been advocated by M. Dumas, who based the suggestion of the composite nature of the elementary atoms on certain relations of atomic weights. The composite nature of the chemical elements has also been maintained by Henri Sainte-Claire Deville, and also by Berthelot, who held that the atoms of the elements are the same, and distinguished only by their modes of motion. Professor Schuster, in a paper read before the British Association in 1880, supports the view of the dissociation of the chemical elements. [Footnote 67: _Proc. Roy. Soc._ vol. xxviii. p. 160.]