Refraction of Quartz produced hy Electrical Force. 103 
an axis of maximum and an axis of no piezoelectricity, then 
the plane of bisection was inclined to the direction of pressure 
at an angle which at any rate did not differ much from 45° ; 
a more accurate determination of this angle was not possible 
with the apparatus at my disposal. 
The maximum piezoelectricity was found at the ends of 
the diameter at right angles to the plane of bisection — conse- 
quently, in the last mentioned case, at the ends of a diameter 
at right angles to the axis and inclined at 45° to the direction 
of pressure — that is, at the ends of an axis of no piezoelectricity. 
In the three following series of experiments the quan- 
tities of electricity produced were so small that I cannot 
regard the results communicated as absolutely certain. I do 
not, however, believe that any different result would be ob- 
tained by the use of more delicate apparatus; but such a con- 
firmation would be desirable. 
(6) A direction lying between an axis of maximum piezo- 
electricity and the principal axis was chosen as the direction 
of pressure. The sphere was divided into two oppositely 
electrified halves by that plane of no piezoelectricity which 
was at right angles to the plane passing through the direction 
of pressure and the principal axis. The sign of the electricity 
is determined by that of the fields in which the points of 
pressure are situated. If, for example, they lay in fields 1 
and 4, then fields 6, 1, and 2 were positive, and fields 3, 4, and 
5 were negative. 
(7) If the pressure was exerted in any direction different 
from those already considered and not coincident with the 
principal axis, the sphere was always divided into two oppo- 
sitely electrified halves by a plane which in all cases passed 
through the principal axis. 
(8) Increase of pressure in the direction of the principal 
axis produced small quantities of electricity in the six fields, 
the sign of which corresponded to the sign of the fields as 
given above : the points of pressure were not electrified. 
It follows from the above that, whatever the direction of 
pressure, there is never any perceptible evolution of electricity at 
the ends of a diameter parallel to the principal axis. It should 
be mentioned, that in the dry air of a room the piezoelectric 
experiments take place extremely regularly and with certainty. 
An objection may be raised to the results described on the 
ground that they are influenced by induction or possibly by 
surface-conduction of electricity. It is not to be denied that 
these causes, and in particular induction, had a part to play ; but 
I have repeatedly convinced myself that these results remained 
qualitatively the same when such disturbing causes were 
