192 
THE MEDITERRANEANS NATURALIST 
take up the thread and discuss the conditions 
under whicli this water is absorbed, retained, and 
expelled. 
Let us take an illustration, namely, the solution 
of carbonic anhydride in water itself. Carbonic 
anhydride is, at the normal temperature and pres- 
sure of the atmosphere, a gas; but by either 
increasing the pressure or lowering the temperature 
it may be reduced to the liquid or solid state. 
The water of volcanoes, at the normal pressure 
of the air and the temperature of lava, is a gas; 
and, like carbonic anhydride, may be rendered 
liquid or solid by increasing pressure or lower- 
ing temperature. By removing either of these 
secondary conditions the more volatile ma- 
terials in the two cases return to their gaseous 
state. Now, carbonic anhydride in the presence 
of water is much more easily condensed, and dis- 
solves simultaneously in that liquid, the solubility 
proportionally increasing with the pressure. Water 
is equally soluble in molten silicates, as is shown 
by its escape from lava, and its solubility likewise 
increases with the pressure, unless downright 
opposed to known physical laws. Between these 
two cases of gases soluble in liquids there is not 
only a physical, but also a chemical, analogy, for 
in both cases we have to deal with gaseous oxides 
soluble in liquid, ones. 
In the case of the solution of carbonic anhydride 
in water, and in fact, of solutions of all gases in 
liquids, we find that the quantity of gas absorbed 
increases with the pressure, provided the liquid 
does not solidify (as exhibited in the spitting of 
silver). We find also that the pressure remaining 
fixed, an increase of temperature has a tendency 
to reconvert the condensed, or, more properly, 
dissolved gas, again into the gaseous state; or, in 
other words, we find that the tension of such a 
solution increases with the temperature. The 
absorption of oxygen by molten silver, and of the 
same gas and carbonic anhydride by iron and 
steel, as demostrated by Troost, are familiar 
examples of fluids at high temperatures taking up 
gases. It is at the same time evident that the 
critical point of water no longer enters into the 
question, as it is held in solution like C02 in 
water, both occupying volumes much nearer their 
liquid than their gaseous state. 
The conditions under which igneous matter com- 
mences its course towards the surface may. no 
doubt, be very variable, and whether such be due 
to secular cooling of our globe, and consequent 
straining and fracturing of its outer surface, it is 
not our present business to discuss. As already 
stated, we have every reason to believe the vol- 
canic magma, as it exists in its original site, (1) 
contains dissolved in it little, if any, water, al- 
though many hold, on account of Sorby's disco- 
veries, that the fluid portion of the earth's interi m 
is an igneo-aqueous solutions. We must first prove 
that granite, or at least that studied by Sorby 
and others, was not an intrusive rock in porou> 
strata. In other words, it must be proved chat 
granite is the primitive rock cooled without the 
intervention of secondary water. 
It, therefore, on being transferred from great to 
lesser pressure, would only exert that small 
amount of expansion which is proper to its chemi- 
cal components, which would therefore undergo 
no change of state, but remain as liquids under 
normal atmospheric pressure at the earth’s surface. 
In fact, whatever expansion tended to take place 
in transferring the volcanic magma from great 
depths to the surface would be more or less 
balanced by the corresponding loss of heat, and 
consequent tendency to contract as a result of that, 
so that only a change in volume would take place, 
if any, in proportion to the different power of the 
two agencies to accelerate or diminish contraction. 
This theory of the solution of water in lava, and 
not lava in water, is incidentally mentioned by 
the Bev. O. Fisher. (2) 
Extrusion of Igneous Matter through Erg, or 
nearly Dry, Rods to the Surface. — Should such 
| volcanic magma in its native state reach the 
surface, it might overflow without any explosive 
manifestations whatever, and consequently no 
cone of scoria or other fragmentary materials 
would be formed around the exit, and the locality 
of this would be only detected on a plain by the 
possible formation of domes, or mamellons, where 
(?) Whether this forms the centre of our globe, 
a stratum between the nucleus and crust, or e.vist 
as isolated reservoirs, in no tray affects that pan 
of the question, now under discussion. 
(k ) Physics of the Earth's Crust, 1SS1, g. 190. 
