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volumes of steam are a conspicuous feature in all volcanoes. 
A piece of lava which I have microscopically examined from 
the eruption of Vesuvius in 1872 was rendered porous by 
the passage of steam through it. Pumice is a further illustra- 
tion of the same thing; it is simply lava rendered porous by 
steam, and perhaps gases, but chiefly the former. During the 
eruption of Vesuvius in 1872 the escape of steam from the 
lava produced minature volcanoes on the lava stream, and im- 
mense volumes were shot high into the atmosphere direct from 
the crater. The steam cloud is a feature in all volcanic 
eruptions, and on condensations taking place descends as 
rain accompanied by thunder and lightning. Mr Scrope in 
his book on volcanoes, published in 1862, advocated a theory 
that volcanic outbursts are due to the accumulation of steam 
at volcanic centres. Professor Prestwich* has recently dis- 
cussed the agency of water in volcanic eruptions, and in doing 
so fully faces the difficulties which have to be met before Mr 
Scrope’s theory can be accepted. 
We know that as we descend into the interior of the earth 
the temperature increases. In the district around Bristol it 
increases at the rate of 1° F. for every 68 feet descent, but 
the rate would doubtless increase at great depths. In other 
localities the rate of increase is sometimes greater, and in 
some instances less. We need not, however, discuss that 
problem, it is enough for us to know that there is an increase 
of temperature as we descend into the earth. It is, then, clear 
that a depth would be reached at which water should be raised 
to the boiling point, and deeper still at which minerals would 
be in a state of fusion. In calculating these depths, however, 
we must allow for pressure; water in becoming steam requires 
to expand, and at the depth at which a temperature of 212° F. 
would be reached (the boiling point of water), the fluid would 
be subjected to immense pressure in the effort to pass into 
steam. The same with mineral substances, but of course their 
fusion temperatures vary. We must not now enter into a dis- 
cussion on the condition of the interior of the earth, suffice it 
* Proceedings Royal Society, No. 246, 1886. 
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