434 
series of fractures could have originated simultaneously, 
but it is, to say the least, much less difficult to under- 
stand if the development of strain over the whole of 
the central area had been sudden, or at any rate rapid. 
Then there is another point to be noticed, that in 
the central region the successive isoseists lie close 
together, while in the outer fringe they lie far apart ; 
thus the distance separating the isoseists of ten and 
seven degrees, covering a range of three degrees of 
intensity, varies from 6 to 20 miles on either side 
of the San Andreas fault, while in the outer regions 
a similar range of three degrees covers from 120 to 
250 miles. ‘The close-set isoseists of the central region 
indicate a shallow origin, and such is proved by the 
San Andreas rifts, where the origin reached the surface 
of the ground; the widely set outer isoseists.similarly 
indicate a deep-lying origin: and so we reach, the 
conclusion that the earthquake origin was of a two- 
fold nature, the great violence in the central region 
being due to fractures and displacements close to, or 
at a comparatively shallow depth below, the surface, 
and that these fractures were the secondary result of 
a more deep-seated disturbance or bathyseism. 
Having reached this conclusion there remain two 
questions of importance, what is the depth, and what 
is the nature of this bathyseism? As to the depth, 
the study of a remarkable, though only feeble, earth- 
quake which affected northern Italy on August 7, 
1895, has led me to conclude that the ultimate origin 
lay at 100 to 150 miles below the surface ; but the best 
indications are to be had from the long-distance records 
of disturbances, which need not necessarily have been 
great earthquakes, in the ordinary sense of the words. 
From these Dr. L. Pilgrim, in 1913, deduced the 
conclusion that the origin of the disturbance, in the 
case of the Californian earthquake, lay at a depth of 
about roo miles, and, more recently, a similar method 
has been developed in this country by Prof. H. H. 
Turner, who has shown that the long-distance records 
indicate depths of origin ranging from fifty to three 
hundred miles below the surface of the earth. Now 
it seems fairly well established that earthquakes of 
quite shallow origin do not give rise to distant records, 
even when very violent in the place where they are 
felt, and it is probable that the disturbance recorded 
by these distant seismographs is not the superficial 
destructive earthquake, but the bathyseism. 
Next comes the question of the nature of the bathy- 
seism. That it must be in some way accompanied 
by a change in bulk of the material underlying the 
NATURE 


[Marcu 31, 1923 
central area of the earthquake, seems clear, in some 
cases at least. Fracture such as is sufficient to explain 
most of the features of the surface shock seems out 
of the question, for the depths place it in the region 
of what it is nowadays the fashion to call the astheno- 
sphere, that is, a part of the earth which is weak and 
plastic against stresses of long duration ; but as regards 
change of bulk, recent researches have indicated one 
very likely mode in which it might be brought about. 
It is known that the foundation rocks of the outer 
crust are everywhere composed of an aggregate of 
crystalline minerals, the detailed study of which shows 
that the material must once have been in a condition 
analogous to that of fusion, from which it has solidified 
by cooling to its present condition. Further, it has 
been shown that the same original magma may crystal- 
lise out as quite different mineral aggregates, differing 
in density, and therefore in volume, by anything up 
to 20 per cent. The exact conditions which determine 
the passage from one form of chemical grouping to 
another are not known in detail, but it is probable that 
in each case there is some critical limit of temperature 
and pressure which determines it. If there were, in 
the interior of the earth, a mass of material near this 
critical limit, a small change of pressure or temperature 
might bring about a change of chemical combination, 
and with it a greater or lesser change of bulk, which, 
transmitted to the upper layers of the earth’s crust, 
would give rise to displacements and distortion. Such 
changes might be unaccompanied by earthquakes, 
if they were slow and gradual, or, if rapid or sudden, 
might give rise to fractures in the surface rocks, of 
greater or lesser magnitude, and covering a larger or 
smaller area, according to the bulk of.the deep-seated 
material undergoing a change of volume. 
Without insisting on this as the nature of the bathy- 
seism, and it is possible that other causes as yet un- 
suspected may also be at work, it is evident that we 
have an explanation which would suffice in the case of 
the larger, and of many of the smaller earthquakes. Yet 
there are some causes, perhaps no inconsiderable fraction 
of the total, in which the whole process leading up to 
the earthquake seems to lie quite close to the surface. 
To these, always small in extent, though sometimes 
of considerable severity, the consideration which I 
have outlined cannot at present be applied; in part 
they must be due to quite different causes, the con- 
sideration of which is not without interest, but this 
interest only arises after more extended and technical 
study than could be presented, even in outline. 
Hydrogen Ion Concentration. 
By Prof. A. V. Hitz, F.R.S. 
(aT solutions are capable of conducting 
electricity, although their separate pure com- 
ponents are themselves incapable, or capable only to 
a slight degree, of so doing. This conductivity is 
attributed to the “ionisation” of the dissolved body, 
that is, to the splitting up of its molecule into two or 
more parts, some carrying a positive and others a 
negative charge, the resulting “ions” being capable 
of migration under an imposed electric field, and so 
giving to the solution the power of carrying a current. 
The electrically neutral molecule breaks up into (a) a 
NO. 2787, VOL. 111] 
negatively charged part, containing an excess of 
electrons which lend it its negative charge, and (b) a 
positive portion with a deficit of electrons, this deficit 
resulting in an equal positive charge. These positive 
and negative ions attract one another, as do all positive 
and negative charges, and are separable only if their 
mutual attraction be small enough to be overcome by 
the inter- and intra-molecular dynamic forces (not yet 
properly understood) tending to their separation. 
The attraction between two charges is far greater if 
they be separated by some media than by others, to a 

