Electric Absorption of Crystals. 419 



These Tables seem to prove beyond question that calcite in 

 clear crystal has no electric absorption. Quartz seems to 

 have about -^ that of glass; but we have remarked that quartz 

 is not a good substance to test the theory upon. 



Some experiments were made with cleavage-plates of sele- 

 nite, which are always more or less imperfect, as the laminae 

 are very apt to separate. These gave, however, effects about 

 5 or £ those of glass. 



In order to test still further the absence of electric absorp- 

 tion in calcite, the electrometer was rendered very sensitive, 

 and the calcite plates were tested with gradually increasing 

 charges, from that which in glass gave 200 millim. after 1 

 second contact, up to the maximum charge (ten sparks of the 

 unit-jar) which the condensers were capable of carrying. In 

 these trials, the calcite still showed no effect, even with 

 30 seconds contact. During these experiments glass was fre- 

 quently substituted for the calcite, to leave no question but 

 that the apparatus was in working order. 



It is to be noted that the relative effects of the quartz and 

 the glass were different for dried plates and plates exposed to 

 the atmosphere. This was possibly due to the glass being a 

 better insulator, and thus retaining its charge better when dry 

 than in its ordinary condition. 



IV. 



Thus we have found, for the first time, a solid which has no 

 electric absorption ; and it is a body which, above all others, 

 the theory of Clausius and Maxwell would indicate. The 

 small amount of the effect in quartz and selenite also confirms 

 the theory, provided that we can show that in the given piece 

 of quartz some molecules of right-handed quartz were mixed 

 with the left; for we know that the theoretical conditions for 

 the absence of electric absorption are rarely satisfied by lami- 

 nated substances like selenite or mica. If the theory is con- 

 firmed, the apparatus here described should give the only test 

 we yet have of the perfect homogeneity of insulating bodies ; 

 for any optical test cannot penetrate, as this does, to the very 

 structure of the molecule. 



