Or the Thermoelectric Properties of Quartz. 195 
advanced further, the pressure exerted by the outer layers 
does not increase any more, but begins to diminish ; but then 
the sign of the piezoelectricity evolved changes also, and 
the electricity produced during the first period becomes more 
and more weakened. During this second period the sphere 
becomes continually less strongly electric; and it may hap- 
pen, especially if a part of the electricity produced has dis- 
appeared by conduction during the first period, that the all 
but cold sphere shows an electrification which is the opposite 
of that found when the cooling commenced. 
These conclusions I have been able to verify repeatedly 
by observing the electricity appearing upon a sphere of 
quartz freely suspended, during cooling. Herr Hankel* has 
also observed the phenomena just described with quartz 
crystals. He, however, calls the electricity which makes its 
appearance at first actinoelectricity, and that remaining at 
the last thermoelectricity. 
By heating a quartz sphere as uniformly as possible, I have 
obtained phenomena altogether analogous to the preceding; 
but the electricities are now of the opposite sign. 
If we observe that the outer layers which receive the heat 
first exert upon the inner ones a radially directed tension, and 
that, by increase of a tension exerted upon the quartz, the 
same piezoelectricity is produced as by the decrease of pres- 
sure acting in the same direction, the explanation becomes 
easy. 
Local cooling of a previously heated crystal by means of a 
cold stream of air directed against the crystal produces an 
energetic evolution of electricity at the points cooled, if these 
points did not lie exactly in a plane of no piezoelectricity; 
the nature of the resulting electricity was the same as that 
which would be produced at the same place by the increase of 
a pressure exerted there in the direction of a diameter. Local 
heating by a current of warm air produced, on the other hand, 
electricity of the opposite kind. In the first case we have a 
rapid increase of the pressure exerted by the outer layers upon 
the inner ones; in the second case, where the outer layers tend 
to raise themselves above the inner ones, the resulting tension 
increases very rapidly. I therefore consider the electricity 
produced to be simply piezoelectric. The electricity which 
Herr Friedel observed to be produced by placing a heated 
metal ball upon a quartz crystal is identical, both in sign and 
in mode of production, with that produced by a hot current of 
air: it is therefore piezoelectricity. Herr Friedel calls it 
thermoelectricity; Herr Hankel actinoelectricity. 
* Hankel, ' Electrical Researches,' Abhandlung 15, p. 530. 
Q2 
