On the Electric Absorption of Crystals* 415 



"Note " referred to, one of us has shown that in other forms 

 of condenser there can be electric absorption even in the case 

 of homogeneous bodies. Hence the problem was to test the 

 electric absorption of a crystal, in the case of an infinite plate 

 of crystal with parallel sides. The considerations with regard 

 to the infinite plate were avoided by using the guard-ring prin- 

 ciple of Thomson. 



The crystals which could be obtained in large and perfect 

 plates were quartz and calcite. These were of a rather irre- 

 gular form, about 35 millim. across and millim. thick, 

 and perfectly ground to plane parallel faces. There were two 

 quartz plates cut from the same crystal perpendicular to the 

 axis, and two cleavage-plates of Iceland spar. There were also 

 several specimens of glass ground to the same thickness; the 

 plates were all perfectly transparent, with polished faces. 

 Examined by polarized light, the quartz plates seemed per- 

 fectly homogeneous at all points except near the edge of one 

 of them. This one showed traces of amethystine structure at 

 that point; and a portion of one edge had a piece of quartz of 

 opposite rotation set in; but the portion which was used in 

 the experiment was apparently perfectly regular in structure. 

 The fact that there are two species of quartz, right- and left- 

 handed, with only a slight change in their crystalline struc- 

 ture, and that, as in amethyst, they often occur together, 

 makes it not improbable that most pieces of right-handed 

 quartz contain some molecules of left-handed quartz, and vice 

 versa. In this case quartz might possess the property of 

 electric absorption to some degree. But Iceland spar should 

 evidently more nearly satisfy the conditions. It is unfortu- 

 nate that the two pieces of quartz were not cut from different 

 crystals. 



This reasoning was confirmed by the experiments, which 

 showed that the quartz had about one ninth the absorption of 

 glass; but that the Iceland spar had none whatever, and is 

 thus the first solid so far found having no electric absorption. 

 Some crystals of mica &c. were tried; but calc spar is the only 

 one which we can say, a priori, is perfectly homogeneous. 

 Thus mica and selenite are so very lamellar in their character, 

 that few specimens ever appear in which the laminae are not 

 more or less separated from one another; and thus they should 

 have electric absorption. 



II. 



In the ordinary method of experimenting with the various 

 forms of Leyden jar, there are, besides the residual discharge 

 due to electric absorption in the substance of the insulator, 



2H2 



