614 JOHN JOHNSTON AND PAUL NIGGLI 
mortar of minerals containing water or even of substances con- 
taining water of crystallization results in a loss of water;’ thus 
apophyllite on grinding lost about 1 per cent of water (Thugutt), 
MgSO,7H.O lost about 2.5 per cent H,O (Blecker). Moreover, 
Johnston has found? that some carbon dioxide can be driven off 
from a substance as stable as calcium carbonate by grinding it in 
a mortar for a few minutes; in this case the grinding lowers the 
temperature at which the calcium carbonate exhibits an appre- 
ciable pressure of carbon dioxide by some 500”. 
This behavior is in thorough accordance with the standpoint 
adopted with regard to the effects of unequal pressure. Applica- 
tion of precisely similar reasoning leads to a formula entirely 
analogous to equation V, namely: 
Cl lena 
Boe vo 
where 7, is the temperature corresponding to a given pressure of 
the vapor phase, AZ the heat of reaction, and V a (specific) volume, 
‘which practically will be intermediate between that of the original 
solid and the volume change accompanying the reaction. Further- 
more, we see that the unequal pressure required to produce an 
appreciable pressure (p) of the vapor phase at a given tempera- 
ture would be approximately proportional to Q log Ty,’ T, being 
the temperature at which the system in the unstressed condition 
would exhibit the vapor pressure p, and Q the heat of dissociation; 
- and this conclusion is roughly borne out by the observations hitherto 
recorded. 
Analogous considerations apply to reactions such as RCO;+ 
SiO, > RSiO,;+CO,; the general conclusion being that unequal 
stress will cause reactions between solids accompanied by the devel- 
* Mauzelius, Sveriges Geol. Undersékning Arsbok, I (1907), No. 3; W. F. Hille- 
brand, “Influence of Fine Grinding on the Water and Ferrous Iron Content of Minerals 
and Rocks,” Jour. Am. Chem. Soc., XXX (1908), 1120: Chem. News, XCVIII (1908), 
205, 215, who refers to some earlier observations; S. J. Thugutt, Centr. Min. Geol. 
(1909), p. 677; I. B. Blecker, Chem. News, CI (1910), 30. 
2 A separate note dealing with this question is in course of preparation. 
3 Since from one substance to another there is comparatively little variation of V, 
but large variations of Q and Tp. 
