ELECTRIC ABSORPTION OF CRYSTALS 205 



none which we can regard as perfectly homogeneous along any given 

 line through them, except crystals. The theory would then indicate 

 that crystals should have no electric absorption; and it is the object of 

 this paper to test this point. The theory of both Clausius and Max- 

 well refers only to the case of a condenser made of two parallel planes. 

 In the ' Note ' referred to, one of us has shown that in other forms 

 of condenser there can be electric absorption even in the case of homo- 

 geneous bodies. Hence the problem was to test the electric absorp- 

 tion 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 principle of Thomson. 



The crystals which could be obtained in large and perfect plates 

 were quartz and calcite. These were of a rather irregular form, about 

 35 millim. across and 3 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. Exam- 

 ined by polarized light, the quartz plates seemed perfectly homo- 

 geneous 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 

 structure, 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 unfortunate 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 per- 



s [There is a gap in the printed article. On examination of the various plates if 

 the Physical Laboratory of the Johns Hopkins University, some have been found on 

 about 2 mm. thickness, which are probably those used in this research.] 



