The various kinds of matter we are acquainted with are commonly called the elements. These when combined in various ways exhibit characteristic phenomena which depend upon the kinds of matter, the structure and motions which are involved. There are some seventy different kinds of this elemental matter which may be identified as constituents of the earth. Many of the same elements have been identified in the sun and stars, such for instance as hydrogen, carbon, and iron. Such phenomena lead us to conclude that the kinds of matter elsewhere in the universe are identical with such as we are familiar with, and that elsewhere the variety is as great. The qualities of the elements, within a certain range of temperature, are permanent; they are not subject to fluctuations, though the qualities of combinations of them may vary indefinitely. The elements therefore may be regarded as retaining their identity in all ordinary experience. THE ETHER IS HOMOGENEOUS. One part of the ether is precisely like any other part everywhere and always, and there are no such distinctions in it as correspond with the elemental forms of matter. 4. MATTER IS ATOMIC. There is an ultimate particle of each one of the elements which is practically absolute and known as an atom. The atom retains its identity through all combinations and processes. It may be here or there, move fast or slow, but its atomic form persists. THE ETHER IS NON-ATOMIC. One might infer, from what has already been said about continuity, that the ether could not be constituted of separable particles like masses of matter; for no matter how minute they might be, there would be interspaces and unoccupied spaces which would present us with phenomena which have never been seen. It is the general consensus of opinion among those who have studied the subject that the ether is not atomic in structure. 5. MATTER HAS DEFINITE STRUCTURE. Every atom of every element is so like every other atom of the same element as to exhibit the same characteristics, size, weight, chemical activity, vibratory rate, etc., and it is thus shown conclusively that the structural form of the elemental particles is the same for each element, for such characteristic reactions as they exhibit could hardly be if they were mechanically unlike. Of what form the atoms of an element may be is not very definitely known. The earlier philosophers assumed them to be hard round particles, but later thinkers have concluded that atoms of such a character are highly improbable, for they could not exhibit in this case the properties which the elements do exhibit. They have therefore dismissed such a conception from consideration. In place of this hypothesis has been substituted a very different idea, namely, that an atom is a vortex-ring[1] of ether floating in the ether, as a smoke-ring puffed out by a locomotive in still air may float in the air and show various phenomena.