[ 194 ] 
XXVIII. On the Thermoelectric, Actinoelectric, and Piezoelec- 
tric Properties of Quartz. By W. C. Rc^tgen*. 
a^HE last of the experiments described in rny second com- 
munication on the optical behaviour of quartz in the 
electrical field induced me (as already mentioned f) to inves- 
tigate experimentally the electrical properties of quartz, espe- 
cially its thermoelectric and actinoelectric properties. In this 
investigation I very soon arrived at the conclusion that it is 
possible to refer to a common cause the evolution of electricity 
brought about in very different ways, whether by conduc- 
tion of heat, radiation, or alteration of pressure — namely, to a 
change in the strains produced in the crystal by any means. 
I therefore consider it unnecessary to distinguish the three 
kinds of electricity named as differing from each other in their 
mode of production, and should propose to retain only the 
name piezoelectricity, if further investigation should show 
that the method of explanation indicated is applicable in all 
cases and sufficient. I have delayed the publication of this 
view and of these experiments, in the first place because the 
experiments were not altogether complete, and, secondly, 
because my theory is so entirely at variance with that pro- 
posed by so experienced and skilful an experimenter as 
Herr Hankel. It seems to me now, however, not possible to 
wait longer ; and in the following I briefly describe the most 
important of my experiments. 
I think we may conclude, from the electro-optical experi- 
ments described, that increase of a uniform surface-pressure 
exerted all over a quartz cylinder or quartz sphere produces 
such an evolution of electricity that the surface is divided by 
the three planes of no piezoelectricity into six electrical 
fields, which have the same position and sign as those ob- 
tained by the method of increase of pressure in one direction, 
described on p. 100 of my second paper (Phil. Mag. August 
1883> 
If we bring a non-electrified and hot sphere, having a 
uniform temperature all over, into a colder space (so that it 
can cool uniformly), the external layers, which cool first, will 
exert a pressure upon the inner ones everywhere in a radial 
direction, which will increase rapidly at first : consequently 
during this period, which we will call the first, we shall have 
the same distribution of electricity upon the sphere which 
corresponds to an increase of surface-pressure mechanically 
exerted upon it. After some time, when the cooling has 
* Translated from a separate impression from the Ber. der Oberh. Ges. 
fiiT Xatur-imd Heilkunde, communicated by the Author. 
t Phil. Mag. August 1683, p. 109. 
