502 JOHN JOHNSTON AND PAUL NIGGLI 
transmit pressure become exceedingly viscous.* A non-uniform 
compression may always be resolved into a shearing stress and a 
(smaller) uniform pressure, the former being the preponderating 
factor in producing the results observed. Now a consistent account 
of all the experimental work can be given if we make the plausible 
assumption that the shearing stress acts on the solid phase, but not 
on the liquid phase; in accord with this, in non-uniform compres- 
sion the liquid (fluid) phase would be subject to less pressure than 
the solid phase, whence the name “unequal pressure” chosen to 
designate this type of compression.” 
EFFECT OF UNIFORM PRESSURE ON MELTING-POINTS 
Thermodynamical considerations lead directly to the differential 
equation Pee 
an ED (I) 
which expresses the change of melting-point (dT) with change of 
pressure (dp) in terms of T, the absolute temperature of melting, 
dV, the volume change and AH, the heat change which accom- 
panies melting of the substance at T. ° Before this equation can be 
applied it must be integrated, since we are dealing with finite 
changes. 
In order to integrate this equation rigorously it is necessary to 
know how dV and AH vary with pressure and temperature. The 
exact magnitude of these variations is not known in general; but, 
fortunately, their effect is slight and may for most practical pur- 
poses be neglected. We may consequently use the following form 
of the equation to calculate the change of melting-point (AT) 
produced by a change of pressure (AP,, expressed in atmospheres’) : 
AT, _ T(Vi—Vs) (II) 
[NE eo 2xo) (O) 
For instance, at 20,000 atm. and ordinary temperatures, even gasoline becomes 
quite viscous (approximately, perhaps, of the consistency of vaseline). 
2 A fuller discussion of this topic, together with many references to previous work, 
will be found in a paper by Johnston and Adams, Am. Jour. Sci. (4), XXXV (1913), 
205; Z. anorg. Chem., LXXX (1913), 281; cf. also postea. 
3 That is, true atmospheres (1,033 g. per sq. cm.). 
