452 PROCEEDINGS OF THE AMERICAN ACADEMY. 



point of 2 mm. pressure, when it is generally considered of the greatest 

 couductibility. 



3. There are anode X-rays as well as cathode X-rays, and these rays 

 exhibit all the peculiarities of the cathode rays ; such as the excitation 

 of fluorescence, phosphorescence, and the peculiar action of breaking 

 down the initial resistance of air and other gases. 



4. The X-rays can be distinctly produced with an electromotive force 

 of 10,000 volts, and there are indications of them at a voltage of 5,000. 



5. Electrostatic induction is an important phenomenon in that of 

 X-rays. Experiments indicate that the X-rays are evidence of an electro- 

 magnetic disturbance, to which lines of electrostatic form are set up in 

 the medium, and that the disturbance therefore travels with the velocity of 

 light, and is accompanied by molecular excitation, which however is only 

 one of the concomitants of the electromagnetic disturbance. 



6. The mechanism, therefore, of the production of the X-rays appears 

 to be a setting up of electrostatic lines of induction along which the 

 electromotive force or difference of potential acts, and a production of an 

 electromagnetic wave or impulse. The stress in the medium reduces its 

 initial resistance, and the X-rays' radiation becomes less and less ener- 

 getic after a certain interval the longer the Crookes tube is excited ; for 

 the increased couductibility of the medium persists after each pulse of 

 such radiation, and therefore the difference of potential between the 

 terminals speedily drops, or does not rise to the full value which the 

 induction machine is capable of developing. 



7. The behavior of highly rarefied media to powerful electric stress 

 is analogous to that of elastic solids to mechanical stresses. A so 

 called vacuum which acts like an insulator when submitted to an electro- 

 motive force capable of producing a spark of eight inches in air 

 (approximately two hundred thousand volts) breaks down under the 

 stress of three million volts. A single discharge with this voltage through 

 highly rarefied media produces the X-rays in such a powerful manner 

 that a photograph of the bones of the hand can be taken in one millionth 

 of a second. During the discharge the apparent resistance of the rarefied 

 medium is only a few ohms. In this case the medium seems completely 

 to lose its elasticity so to speak, and is ruptured. The elastic solid 

 analogy seems to explain the behavior of highly rarefied media toward 

 powerful electric stresses. The generation of the X-rays is accompanied 

 by a powerful electrostatic stress which breaks down the so called elasticity 

 of the medium. The question of the electrical conductivity of the ether, 

 it seems to me, can be studied best from the elastic solid point of view. 



