172 hertz's researches on electrical waves. 



(3) When tbe plates A A' rested on the asphalt block the oscillation 

 period of the i)riiuary was increased, as shown l>y the fact that the 

 period of B had to be slightly increased in order to obtain the max- 

 imum sparking" distance. 



(4) When the apparatus was moved gradually away from the block 

 its action steadily diminished without changing its character. 



(5) The action of the block could be compensated by bringing the con- 

 ductor C over the plates A A', while they rested on the block, the null 

 points being brought back to a and a' when C was at a height of 11 

 centimeters above the j^lates. When the upper surface of the asphalt 

 was 5 centimeters below the plates, compensation was obtained when C 

 was placed at a height of 17 centimeters above them, showing that the 

 action of the di-electric was of the order of magnitude which had been 

 anticipated. 



The asphalt contained about 5 per cent, of aluminium and iron com- 

 pounds, 40 ])er cent, of calcium compounds, and 17 per cent, of quartz 

 sand. In order to make sure that the observed effects were not due 

 to the conductivity of some of these substances, a number of further 

 experiments were made. 



In the first place the asphalt was replaced by a mass of the same 

 dimensions of the so-called artificial pitch prepared from coal, and ef- 

 fects of a similar kind were observed, but slightly weaker, the great- 

 est displacement of the null i)oints amounting to 19°. Unfortunately 

 this pitch contains free carbon, the amount of which it is difficult to 

 determine, and this would have some conductivity. 



The experiments were then repeated with a conductor, C, of half the 

 linear dimensions of the former one, and smaller blocks of various sub- 

 stances, on account of the great cost of obtaining large blocks of pure 

 materials. The substances used were asphalt, coal-pitch, i^aper, wood, 

 sandstone, sulphur, paraffine, and also a fluid di-electric, namely petro- 

 leum. With the smaller apparatus it was not possible to obtain quan- 

 titative results of the same accuracy as before, but the effects were 

 of an exactly similar character, and left little room for doubt of the 

 reality of the action of the di-electric. 



The results might possibly be supposed to be due to a change in 

 the distribution of the electro-static e. m. f. in the neighborhood of 

 the dielectric, but in the first place Dr. Hertz states that he has been 

 unable to explain the details of the observations on this hypothesis, 

 and in the second place it is disproved by the following experiment : 



The smaller apparatus was placed with the line rs on the upper near 

 corner of one of tlie large blocks, in which position the di-electric was 

 bounded by the plane of the plates A A' and the perpendicular plane 

 through rs, both of which are equipotential surfaces, so that if the 

 action were electro-static no effect should be produced by the dielectric. 

 It was found however to produce the same effect as in other positions. 

 It might also be supposed that the effects were due to a slight conduc- 



