1308 
rature is increased But, whereas mean forces occurring at the 
thermical pressure could be deducted at once from the energy, this 
time this is not the case, and thus here it will be necessary to 
calculate the mean values e’, e’, separately. 
This calculation of which here only the general course will be 
indicated, is performed a little more easily for another case, i.e. for 
the case that the given deformation at the absolute zero is 
t= ¢; Gn Car 6, ber eee 
Then only tensions X;, Y, and Z: occur and the mean values 
will have to be determined. 
These mean values can be found by considering the progressive 
waves in the rhombic crystal into which the body has changed by 
the deformation. 
The total change can be characterized as follows. First the energy 
in the isotropic body is divided equally in all directions over the 
waves of the same frequency; for the crystal this is not the case. 
In the second place in the isotropic substance there are longi- 
tudinal and transversal waves; with the crystal the direction of the 
displacement is no longer so simple. Now e,, e, and e, are small 
quantities, the change is therefore for both cases small; e.g. from 
the longitudinal wave arises a wave the elongation of which has a 
small inclination with respect to the wave-normal. As the effects 
are so small we are able to determine the influence on the averages 
separately. Indeed we may in calculating the influence of the new 
division of energy overlook the ‘‘declivity” of the waves, hence we 
may substitute the direction in the isotropic case for that of the 
vector of displacement. When we examine however the influence 
of the ‘declivity’, we can take into account the division of the 
energy for an isotropic body, i.e. the homogeneous distribution over 
the directions. 
As has been said above, we do not intend to reproduce here the 
calculation, but are going to show only how the elastie constants 
of the rhombic erystal are expressed in the magnitudes ev, e, e,. 
We introduce the notation of Vorer sorthat C,, €, C,3 Ca3Ca8 Coa Cas 
CysCg, are the constants of the rhombic crystal, i.e. the coefficients 
of e,?,e,e, etc. in the energy. When the strain at the absolute 
zero is represented’ by @,,¢@,,@,,0,0,0, and the arbitrary strain 
which is superposed on it by e', é, ee, e;e',, then the terms with 
C, D and E give quadratic parts in €, etc. viz. 
