514 THE POPULAR SCIENCE MONTHLY. 



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 spe- 

 cial 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 per- 

 haps best to confine our attention at first to the structure of mat- 

 ter rather than its nature. The properties and behavior of mat- 

 ter 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 * regards as a proper test of a ma- 

 terial 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/' f But Prof. Dolbear 

 has previously announced as his criterion of matter, the posses- 

 sion 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 



* Matter and Motion. f Matter, Ether, and Motion. 



