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lava without recognizing this fact, and the numerous crystals 
contained both in the solid lava and in the small holes, once 
perhaps filled with gas or steam, afford ample proof of the 
nature and extent of the force. But it is the soft spongy tuff 
that is most rapidly affected. Directly the first violence 
of an eruption is over, and after lava has been poured out, 
the eruptive forces are once more kept in, pressed down as 
before by the weight of the whole overlying material, what- 
ever that may be. While the lava is cooling — which it must 
do very slowly, some portion of these gases continues to 
escape, and immediately a6ts on the material through which 
it passes. It dissolves out the alkalies and the iron, causing 
them to enter into new combinations. For a time the gases 
that issue are intensely hot and distinctly acid. By degrees 
they change and even become alkaline, ammonia being 
emitted. They generally (perhaps always) contain free 
hydrogen and nitrogen, and at a certain period sulphuretted 
hydrogen seems to be the chief product. Towards the last, 
carbonic acid gas is given off almost in a pure state. It 
would take too long and involve details and statements too 
purely technical to explain minutely the course of proceeding 
and the resulting salts, nor am I indeed sufficiently well- 
informed on chemistry to venture a complete explanation. 
It is sufficient to say that various aluminous salts, several 
ammoniacal salts, chloride of sodium or common salt, 
chloride of iron, and occasionally sulphates of iron and 
copper, sulphuret of arsenic, and some rare salts, in which 
sulphur is the chief ingredient, are produced and destroyed, 
one set succeeding another with great regularity and rapidity. 
Thus are produced those curious and exquisitely beautiful 
appearances on the walls of a crater now or very recently in 
activity. In the old craters the yellows are far less common, 
though some still remain. The iron, always an important 
element in colour, becomes then converted into its peroxide, 
and assumes a red tint, which does not afterwards change. 
Examples of this of great beauty, and specimens of many of 
the permanent salts, are to be found, besides many that are 
deliquescent, in the interesting craters of the Monti Rossi, from 
which was erupted the lava- current that reached and partly 
destroyed the town of Catania,, in Sicily, in the year 1669. 
Two hundred years have not sufficed to destroy all the marks 
of change produced by the fumaroles that sent gases through 
the erupted cone of ashes from the red-hot lava that once 
issued on the side of Etna, a few hundred feet below. The 
chemical action has been chiefly confined to this part of the 
cone, and it has helped to harden it, for it is much higher here 
than elsewhere. 
