Arrhenius’ Theory of Physical Causes of Vulcanicity. 173 
NOTICHS OF MEMOTRS.- 
ArrueEnius’ THEoRY or THE Puysicat Causes oF Vutcanicity.! By 
R. H. Rastatt, M.A., F.G.S., Fellow of Christ’s College, 
Cambridge. 
[Note.—As this paper in its original form is very inaccessible to most readers, its: 
importance has perhaps scarcely been appreciated in this country; the following 
short abstract has been made at the suggestion of Professor Bonney, in the hope that 
it may be of use to students of geoloay and others. ] 
(1) Lhe Condition of the Earth's Interior. 
HE conclusions of astronomers and physicists are irreconcilable 
with the hypothesis of the existence of any considerable amount. 
of matter in the liquid form in the interior of the earth. This idea 
rests for the most part on a misconception of the physical properties 
of matter under the enormously high pressures which prevail at great 
depths. Nearly all known substances, with the notable exception of 
water, contract in volume on solidifying, and in consequence their 
melting-point is raised by pressure. The investigations, however, of 
Damien, Barus, and especially of Tammann, have shown that at very 
high pressures this relation no longer holds. Tammann found that. 
the maximum melting-point of dimethylethyl-carbinol is reached 
under a pressure of 4,750 atmospheres, and at higher pressures the. 
melting-point falls; it seems probable that other substances would 
behave in asimilar manner under similar conditions. Hence it appears. 
that at great depths the constituents of the globe must be in reality in 
the gaseous state, since pressure cannot keep them solid or even liquid| 
by raising the melting-point. But the physical properties of gases. 
under such conditions must be very unlike those with which we are. 
acquainted. The true difference between a liquid, a solid, and a gas. 
hes in the rate at which they respectively undergo deformation under- 
stress. The necessary physical constants have been determined by 
Reinganum in the case of isopentane at the highest attainable. 
pressures, and from these it is calculated by extrapolation that at. 
a depth of 1,000 kilometres the compressibility of gaseous isopentane- 
must be about equal to that of steel. The temperature in this case. 
must be far above the critical point, and the material is actually 
gaseous, though its physical properties resemble those which are- 
commonly associated with the idea of a solid. Rock-magma under- 
similar conditions would undoubtedly possess a still lower com- 
pressibility. 
Assuming the temperature-gradient to be 3° C. per 100 metres, the 
melting-point of most rocks, about 1,200° C. is attained at a depth of 
about 40 kilometres, while below this lies a liquid zone. At a still 
greater depth the critical point is passed, and the magma is in the 
gaseous state, though intensely rigid. From the high density of 
the earth as a whole, compared with that of the visible portion, it 
appears that the inner nucleus consists of some metallic substance, 
1 Arrhenius, ‘‘ Zur Physik des Vulcanismus’’ : Geologiska Foreningens i Stockholm 
Forhandlingar, Band xxi (1900), p. 395. 
