PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 501 
sions regarded as tentative and preliminary only. Precisely similar 
remarks apply to the effect on this volume change of pressure alone 
or of pressure and temperature combined. 
The computation of heats of reaction for conditions differing 
from those under which the experimental values were determined 
is subject to altogether analogous limitations, the specific heat, or 
change of heat capacity, being the exact analogue of expansion 
coefficient. In this case indeed we may have—although to be sure 
the experimental data are not very concordant—an instance of 
reversal of the heat effect of a reaction with rise of temperature. 
The heat of transformation aragonite > calcite at 20° is small, 
probably positive and less than 1 cal. Now according to the most 
thorough series of measurements for aragonite and calcite,’ their 
mean specific heats within the range o°—300° (the limit of the meas- 
urements) are respectively 0.2246 and 0.2204; hence the difference 
is 0.004 per degree or about 1.8 cal. for 450°. Therefore the heat 
of transformation at 470° will be 1.8 cal. Jess than it is at 20°, and 
thus a minus quantity; this is in agreement with the observations 
of Lashchenko,? who concluded that this transformation at 470° is 
accompanied by the absorption of considerable heat. 
Variation of effects of compression with its character—There is a 
marked difference in the effect of compression according as it is 
uniform or otherwise. Changes in the physical properties of sub- 
stances are induced by uniform pressure, but disappear when the 
pressure is removed. Non-uniform compression produces perma- 
nent deformation of bodies exposed to it, and therefore causes 
permanent alterations in many of the physical properties of the 
substance. The effects of the latter differ so much from, and so 
far outweigh those of uniform pressure, both in character and 
in magnitude, that it is advisable to treat them separately. 
Uniform pressure is, by definition, the same in all directions; 
in practice, however, this is not altogether easy of attainment at 
really high pressures, because then the liquids generally used to 
t Lindner, Sitzber. physik. medizin. Ges. Erlangen, XXXIV (1902), 217. 
2 Lashchenko, Jour. Russ. Phys. Chem. Soc., XLII (1911), 793. It may be noted, 
however, that Boeke, who used a differential method, concluded that the heat change 
at 470° is less than o.5 cal. (Z. anorg. Chem., L [1906], 246). 
