other. There is no longer any doubt about that, and Maxwell's theory, which rests upon this idea fundamentally, has a strong hold upon modern science and a hold that is growing stronger as research advances.
We know the ether as a vehicle of energy in several forms, and when various agencies are collected into a group of forms of energy there is still the question, "What is energy?" These general problems now engaging the attention of the physicist—viz., the ultimate nature of matter, by which the properties of matter may be accounted for; the nature of the ether and its properties; the mutual relations subsisting between matter and ether, if they are different things; the nature of energy, and whence it arises, and whether it is primarily potential or kinetic—these, in part at least, are not new problems, but they are now approached from new directions, along new ways, and by the aid of new light. Under each of these heads appear numerous special questions, and along all these lines investigators are working earnestly.
The attempt to explain the nature of ether or of matter at once raises the question whether ether is matter. Now, of course, a great deal depends upon the definition of terms, and it is perhaps best to confine our attention at first to the structure of matter rather than its nature. The properties and behavior of matter as it is ordinarily recognized are largely known, the actions and functions of the ether are largely known, and it is only a question of the propriety or possibility of including both in one general view. Clerk Maxwell[1] regards as a proper test of a material substance its ability to contain and transmit energy. He then points out that energy can not exist except in connection with matter; that in the space between the sun and the earth, the luminous and thermal radiations which have left the sun and which have not reached the earth possess energy in definitely measurable amount, and therefore this energy must belong to matter in the interplanetary spaces. On the other hand. Prof. Dolbear stands as an exponent of the views of others who decline so to class the ether when he says: "If, then, the ether fills all space, is not atomic in structure, presents no friction to bodies moving through it, and is not subject to the law of gravitation, it does not seem proper to call it matter."[2] But Prof. Dolbear has previously announced as his criterion of matter, the possession of the property of gravitative attraction. On such grounds we may concede each view to be correct, but we are brought at once to the old question, "What is matter?" It is the view of some that, with the present limitations of intellect, it is beyond
