22S Scientific Intelligence. 



producing the waves, no action between them could be detected, 

 the vei'tically produced waves not being picked up by the hori- 

 zontally placed receiver. When the two pieces of apparatus were 

 placed parallel to each other, and a wooden cable, with a number 

 of insulated metallic wire rings wrapped round it was placed in 

 the path of the electrodynamic waves, it produced the same effect 

 as does a tourmaline plate on polarized light. When the wires 

 were vertical — that is to say, parallel to the exciting apparatus — 

 the action was not propagated through the cube ; but it was, on 

 the other hand, when the wires were horizontal. When the re- 

 ceiver with its mirror was placed horizontally, so that it did not 

 record any action on reaching it, and the wire arrangement de- 

 scribed above, was placed in the path of the waves, no change 

 took place in the receiver when the wires on the cube were either 

 vertical or horizontal, but the receiver was affected when the 

 wires were placed at an angle of 45°. The laws of reflection of 

 electrodynamic waves at metallic surfaces were found to be the 

 same as those for the reflection of light at plane mirrors. The 

 refraction of pitch for electric waves was found to be l - 68. — 

 Nature, Jan. 17, 1889. j. t. 



10. Resistance of electrolytes. — Professor J. J. Thomson (Royal 

 Society, Jan. IV, 1889) has examined the screening influence of 

 conducting plates upon alternating currents of great frequency, 

 and has deduced thereby the resistance of electrolytes and of 

 graphite. He shows that the screening effect depends on the 

 conductivity and thickness of the plate and upon the frequency 

 of the alternations. The secondary induced currents are confined 

 to the skin of the plate next to the primary, the thickness of this 

 skin varying with the conductivity of the plate and the frequency 

 of the currents. Thus a thin plate of badly conducting material 

 will be efficient with cm-rents of great frequency such as those of 

 the rate 10*8 per second while a thick plate of the best conducting 

 material will not be sufficient to screen off currents of low fre- 

 quency such as those with a rate below 10*2 per second. Thus to 

 measure the resistance of electrolytes it is necessary to have vibrat- 

 ing electrical systems such as those examined by Hertz, whose 

 frequency is of the former class; and if two different plates pro- 

 duce the same screening effect, their thickness must be propor- 

 tional to their specific resistances. He supports Maxwell's theory 

 that the rate of propagation of electrostatic potential is practically 

 infinite, a point called in question by Hertz, and he agrees with 

 Hertz that the rate of propagation of electrodynamic action is 

 finite and measurable. He shows that the rate of propagation of 

 an electromagnetic disturbance through a metallic conductor and 

 through the surrounding dielectric is the same, and this differs 

 from one of Hertz's conclusions. But he also shows that this is 

 not so when the conductor is a dilute electrolyte or a rarefied gas. 

 In such cases there would be interferences and standing vibrations. 

 Hence the striae in so-called vacuum tubes. He also concludes 

 that the relative resistance of electrolytes is the same when the 



