270 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XIV.) 



with a thin coat of shell-lac dissolved in alcohol, to prevent the deposition of damp 

 upon its surface from the air. It was supported upon a small table of shell-lac fixed 



on the top of a stem of the same substance, the latter being of sufficient strength to 

 sustain the cube, and yet flexible enough from its length to act as a spring, and allow 

 the cube to bear, when in its place, against the shell-lac on the inductric ball. 



1691. Thus it was easy to bring the inducteous ball always to the same distance 

 from the inductric ball, and to uninsulate and insulate it again in its place ; and then, 

 after measuring the force in the electrometer (1 181.), to return it to its place oppo- 

 site to the inductric ball for a second observation. Or it was easy by revolving the 

 stand which supported the cube to bring four of its faces in succession towards the 

 inductric ball, and so observe the force when the lines of inductive action (1304.) 

 coincided with, or were transverse to, the direction of the optical axis of the crystal. 

 Generally from twenty to twenty-eight observations were made in succession upon the 

 four vertical faces of a cube, and then an average expression of the inductive force 

 was obtained, and compared with similar averages obtained at other times, every pre- 

 caution being taken to secure accurate results. 



1692. The first cube used was of rocA; crystal-, it was 07 of an inch in the side. It 

 presented a remarkable and constant difference, the average of not less than 197 ob- 

 servations giving 100 for the specific inductive capacity in the direction coinciding 

 with the optical axis of the cube, whilst 93'59 and 93'31 were the expressions for the 

 two transverse directions. 



1693. But with a second cube of rock crystal corresponding results were not ob- 

 tained. It was 0'77 of an inch in the side. The average of many experiments gave 

 100 for the specific inductive capacity coinciding with the direction of the optical 

 axis, and 98*6 and 99'92 for the two other directions. 



1694. Lord Ashley, whom I have found ever ready to advance the cause of science, 

 obtained for me the loan of three globes of rock crystal belonging to Her Grace the 

 Duchess of Sutherland for the purposes of this investigation. Two had such fissures 

 as to render them unfit for the experiments (1 193. 1698.). The third, which was very 



