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A N:o 16) Melting and boiling point of minerals II. 118) 
above (2,5 being the average density of the molten magma 
and the rock) or 376 meters and 661 meters. 
From the preceding calculations and from the boiling 
points recorded in table IV a somewhat remarkable 
conclusion may be drawn. Already at such depths below 
the surface of the earth as 1000 to 1600 meters, the pre- 
vailing pressure suffices to prevent the boiling or vivid 
sublimation even of such comparatively volatile substances 
as cinnabar, realgar, arsenic, tiemannite and auripigment at 
temperatures, Which are below the melting points of common 
rocks. It is only in case the gases formed are able to escape 
through volcanic craters or open fissures in the crust that 
the boiling may set in at a considerable depth below the 
earth's surface. 
The dissociation of a substance, for instance the devolv- 
ing of CO, from CaCOs, is governed by laws similar to 
those governing the change of boiling points with change 
in pressure. The similarity is far less prominent in the case 
of dissociation and some of the very few investigated sub- 
stances show only a little more than half the increase in 
dissociation temperature which may be calculated from 
the corresponding values 'of chlorine. This similarity may, 
however, be applied in order to obtain some conception of 
for instance the pressure which is necessary to keep pyrite, 
FesSsa, stable at the temperature of the molten rock magma, 
800”—1000?. It appears probable that a pressure of 40 to 60 
atmospheres is sufficient lo raise the dissociation temperature 
of pyrite above 800” from its value at normal pressure, 
665” 2). Pyrite may thus crystallize from a molten rock 
magma at depths, greater than 400 to 600 meters below 
the earth's surface. 
The author determined with his apparatus for measuring 
the melting and boiling points ?) the dissociation point at 
ordinary pressure for several of the minerals which dis- 
sociate at comparatively low temperatures: 
1 Allen, Crenshaw and Johnson. The mineral sulphides of 
iron. American Journal of Sc. 33. 235. (1912). 
? Borgström. Öfversigt af Finska Vet.-Soc. Förh. Bd LVII. A. N:o 29. 
(1915). 
