»5o 



SCIENCE. 



[Vol. XV. No. 369 



•energy would represent the difference between the initial and 

 final potential states of the system. 



He quotes Tait as saying, "Every action between two bodies 

 is a stress," and says that "the body and the ether about it 

 are two bodies, and, if they can • act at all upon each other, 

 there will then be a field." But you will recollect that he 

 makes this statement in controversion of mine, that, if there 

 were but ' 'a single mathematical point in space, there could be 

 no stress. ' ' I said nothing at all about there being such a 

 thing as ether in this connection, though I see the necessity of 

 including it, and also the force of his argument ; but I do not 

 think Tait meant to consider the ether as a body in the sense 

 in which Professor Dolbear here uses it. As I understand it, 

 Faraday, Maxwell. Thomson, and I assume Tait also, believe 

 the dielectric to be the active agent, and the conductor the 

 passive agent, in all the phenomena which we are considering. 

 The dielectric, whatever it may be, — the ether, if you will, — 

 is really the seat of the strains which terminate in the two 

 bodies connected. I think Tait used the term in the sense that 

 I have indicated. 



"Perhaps, however, Mr. Perry calls the ether matter, which 

 has not been my habit, and against which 1 was not on my 

 guard when I wrote the statement to which he objects. Until 

 we have some evidence that ether is subject to the law of 

 gravitation, it seems to me to be improper to speak of it as 

 matter. If every particle of matter attracts every other particle 

 of matter, and'if there is no evidence that ether is so attracted, 

 it is not conducive to good terminology to call it matter." 



Let us see what authority we have for considering ether as 

 matter. I believe the weight of opinion is either that the ether 

 is a form of matter or that matter is a form of ether. Sir 

 William Thomson believes that matter is nothing but ether; 

 that it is composed of it. We know this all-pervading medium 

 as ether when it is unorganized. When it is organized into 

 vortex rings, we have the atom and molecule, hence gross 

 matter, as it is usually distinguished. I am of the opinion 

 that Sir William Thomson's theory of matter is the most 

 popular one at present. In 1838 M. Pouillet found that the 

 heat-energy transmitted from the sun to the earth would, if 

 none were absorbed by our atmosphere, raise 1.76 grams of 

 water 1° C. in 1 minute on each square centimetre of the earth 

 normally exposed to the rays of the sun. This is equivalent 

 to [83.5 foot-pounds of energy per second. This figure Sir 

 William Thomson used in determining the probable density of 

 the ether. 



Herschel estimated the stress (elasticity?) of the ether at 

 17x10' pounds per square inch. S. Tolver Preston estimates 

 the probable inferior limit of the tension of the ether at 500 

 tons per square inch, which is much smaller than Herschel' s 

 ■estimate. Young remarks, ' 'The luminiferous ether pervading 

 all space is nob only highly elastic, but absolutely solid." I 

 do net understand the meaning attaching to "solid" here, but 

 it is evidently an attribute of matter. Sir William Thomson, 

 calculating upon the data above referred to, finds the weight of 

 ■a cubic foot of ether to be f xlO--" pounds. Bellini makes it 

 i X 10-13 pounds. M. Herwitz, another investigator, arbitrarily 

 assumes a cubic foot of ether to weigh 10-'^ pounds. 



De Volson Wood treats the ether as if it conformed to the 

 kinetic theory of gases, which, with other assumptions, is 

 equivalent to considering it as gaseous in its nature, and 

 at once compels him to consider it as molecular. He says, 

 "The electro-magnetic theory of light suggested by Max- 

 well (?), as well as the views of Newton, Thomson, Herschel, 

 Preston, and others, are all in keeping with the molecular 

 hypothesis.'" 



Professor Rood succeeded in producing a vacuum of 39o;ooo;ooo of 

 ;an atmosphere. Professor De Volson Wood states, that, even 

 -at this great rarity of the atmosphere, the quantity of matter 

 in a cubic foot of air "would be some 300 million million 

 times the quantity in a cubic foot of ether," and says, that, 

 admitting that the ether is subject to attraction according to 

 the Newtonian law and of compression according to the law 

 of Mariotte, in order to make the density vary sensibly with 



the distance, the attraction of the central body must be some- 

 thing like a million times as great as that of the sun, or have 

 a diameter a million times as large; but, there being no such 

 known body, he concludes that the density and tension of the 

 ether may be considered uniform throughout space ; and he says 

 that the weight of a given volume of it would vary as the 

 force of gravity, and places the weight of a cubic foot of ether 

 at the surface of the sun at 57 x 10-^' pounds, and estimates the 

 pressure on a square foot of the sun of a column of infinite 

 height at 13 x 10-'< pounds. 



Thus we see, that, while no two of these investigators agree 

 in their results, they all agree in ascribing to the ether all the 

 properties of matter, including that of gravity, and I therefore 

 think it no violation of the proprieties to speak of it as though 

 it were matter. 



In regard to the definition of the word "stress," Professor 

 Dolbear quotes Maxwell as follows: "Now, we are unable to 

 conceive of propagation in time except either as the flight of a 

 material substance through space or the propagation of a condi- 

 tion of motion or stress in a medium already existing in 

 space," and says, "Evidently Maxwell did conceive that stress 

 could travel.'' I freely admit that a "condition of stress" 

 may travel, in the sense that a body between the particles of 

 which there exists a stress may travel ; and it seems to me that is 

 what Maxwell means. If he meant what Professor Dolbear 

 thinks he does, why does he says a ' 'condition of stress' ' ? 

 Why not simply ' 'stress' ' ? 



I think Maxwell was probably the first to use the term 

 "stress," but it was in relation to phenomena described by 

 Faraday. In regard to this, Maxwell himself says (vol. i. p. 

 153), "The distribution of stress considered in this chapter is 

 precisely that to which Faraday was led in his investigation of 

 induction through dielectrics." Further, he says, "This is 

 an exact account of the conclusions to which we have been 

 conducted by our mathematical investigation. At every point 

 of the medium there is a state of stress such that there is 

 tension along the lines of force, and pressure in all directions 

 at right angles to these lines." "The expression 'electric 

 tension' has been used in various senses by different writers. 

 I shall always use it to denote the tension along the lines of 

 force, which, as we have seen, varies from point to point, and 

 is always proportional to the square of the resultant force at 

 the point." "The hypothesis that a state of stress of this 

 kind exists in a fluid dielectric," etc. "The state of stress 

 which we have been studying." "If the medium is not a 

 perfect insulator, the state of constraint which we call electric 

 polarization is continually giving away. Tlie medium yields to 

 the electro-motive force, the electric stress is relaxed, and the 

 potential energy of the state of constraint is converted into 

 heat." "In the phenomenon called the electric current, the 

 constant passage of electricity through the medium tends to 

 restore the state of polarization as fast as the conductivity of 

 the medium allows it to decay. Thus the external agency which 

 maintains the current is always doing work in restoring the 

 polarization of the medium which is continually becoming 

 relaxed, and the potential energy of this polarization is con- 

 tinually being transferred into heat." 



I consider the above as perfectly in accord with my state- 

 ments in your issue of Jan 24. 



On p. 257, §642, he specifically defines "stress " as follows: 

 "Hence the state of stress may be considered as compounded 

 of (1) a pressure equal in all directions, (2) a tension along the 

 line bisecting the angle between the directions of the magnetic 

 force and the magnetic induction, (3) a couple tending to turn 

 every element of the substance, ' ' etc. ' 'The stress in this case 

 is therefore a hydrostatic pressure, combined with a longi- 

 tudinal tension along the lines of force," etc. 



But Faraday was the first to conceive of these stresses 

 although I am not sure that he used this term. In his 

 "Experimental Researches," 8249, he says, "With the electric 

 force we have both the static and dynamic state; . . . still 

 there are well-established electric conditions and effects which 

 the words 'static,' 'dynamic,' and 'current' are generally 



