SEPTEMBER 3, 1897. ] 
absolute heat conduction, encountered in 
passing isothermally from the solid to the 
liquid state, when referred to solid conduc- 
tivity is about 15%, and when referred to 
a liquid conductivity is about 15%. Simi- 
larly the change of thermometric conduc- 
tivity under like conditions is an increment 
of 36% and 56% respectively. Now, since 
in most questions relating to thermal flow 
thermometric conduction enters exclusively, 
the importance of this large coefficient is 
obvious whenever a body passes from the 
solid to the liquid state. 
Solid conduction is thus 40 or 50% in ex- 
cess of liquid conductivity for the same body 
at the same temperature and pressure. It is 
reasonable to infer that a corresponding de- 
erement of conduction will accompany any 
rise of temperature of a solid body. Meas- 
urements which have somewhat recently 
been made for relatively small intervals at 
Zurich, at Glasgow and at Harvard, upon 
typical rocks, all bear out this surmise. The 
immediate incentive to these experiments 
was a strong paper by Professor Perry in 
which Lord Kelvin’s estimate of the age of 
the earth is shown to be insufficient for an 
earth in which the interior conductivity is 
systematically greater than the surface con- 
ductivity. Indeed, he deduces the percent- 
age increment of the square root of the 
age of a Perry earth over that of a Kelvin 
earth to be one-fifth of the percentage de- 
erement of conduction for each 100° C. 
So far as the effect of terrestrial tempera- 
ture alone is concerned, the measurements 
just mentioned show that Perry’s correction 
is negative or that Perry’s earth would be 
less long-lived than the 100 x 10° limit of 
years set by Lord Kelvin.* 
To estimate the effect on heat conduction 
of the increase of pressure which accompa- 
nies the increase of temperature with the 
*The text of Kelvin’s recent address at the Vic- 
toria Institute, in which an age of thirty million 
years is maintained, has not yet reached me. 
SCIENCE. 
347 
depth below the surface is a much more 
serious matter. Inthe laboratory, pressure 
experiments are limited to 3,000 or 4,000 
atmospheres; compared with earth pres- 
sures these scarcely amount to a scratch on 
the surface; yet even for this limit the de- 
termination of heat conduction at high 
temperatures is out of the question. A 
tentative method of arriving ata conclusion 
is given by Clarence King in his discussion 
of the age of the earth, the consequences of 
which have been quite overlooked. What 
King endeavored to accentuate long before 
Perry’s contribution to the subject was pre- 
cisely the fact we cannot assume greater 
conductivity for the interior than for the 
surface. Since heat conduction decreases 
isothermally from solid to liquid, it is as- 
sumed that in one and the same substance 
the viscosity could be taken as an index of 
the thermal conduction. Therefore, if tem- 
perature and pressure were made to vary in 
such a way (both increasing) as to leave 
viscosity constant, it was inferred that heat 
conduction would also remain constant. 
Now the isometries or lines of constant vis- 
cosity of a viscous body for variable pressure 
and temperature are much more easily found 
than the isometrics of conduction. In fact, 
it has been shown that a burden of at least 
200 atmospheres would have to be brought 
to bear in order to wipe out the decreased 
viscosity due to the rise of a single degree 
(Centigrade) of temperature. The depth 
at which this ratio is reached, as King 
points out, for a given surface gradient 
of temperature and depth, depends on the 
initial excess of the temperature of the earth 
considered, and on the age of the tempera- 
ture distribution resulting. But no matter 
whether the Kelvin earth with an initial 
excess of 3,900° and an age of 100 x 10° 
years, or whether King’s solid earth with 
an initial temperature of fused platinum 
and 25x 10° years of life be taken, in all 
cases the temperature effect predominates 
