287 
M. Gay Lussac’s Reflections on Volcanoes. 
stances highly combustible, we still want precise observations to 
enable us to appreciate their action in volcanic phenomena. It 
would be necessary to know the nature of the vapours exhaled 
by several volcanoes ; for the cause which produces their activi- 
ty being assuredly the same, the product, which would be com- 
mon to all, might throw light upon it* All the other products 
would be accidental ; that is to say, they would be owing to the 
action of heat upon the inert matters toward which the volcanic 
focus might extend. 
The great number of burning volcanoes diffused over the sur- 
face of the earth, and the still greater number of mineral masses 
which bear evident marks of their ancient volcanic origin, should, 
in a general point of view, make us consider the bounding stra- 
tum of the earth as a crust of scoriae, beneath which there exist 
a great number of foci, of which some are extinct, while the 
others are in a state of ignition. And, what is well calculated 
to surprise us is, that the earth, so many ages old, still preserves 
an intestine power which raises mountains, overthrows cities, and 
agitates the whole mass. 
The greater number of mountains, on issuing from the bowels 
of the earth, must have left vast cavities, which have remained 
empty, at least they have not been filled with water. But it was 
in a very erroneous way that Deluc, and many other geologists, 
made use of those empty spaces which they imagined to be pro- 
longed in the form of long galleries, for propagating earthquakes 
to a distance. 
An earthquake, as Dr Young has very properly observed, is 
analogous to an undulation in the air. It is a very strong so- 
norous wave, excited in the solid mass of the earth, by an 
agitation which is propagated with the same celerity as sound. 
What is surprising in this great and terrible phenomenon of na- 
ture, is the immense extent to which it is felt, the ravages which 
it produces, and the power of the cause which excites it. But 
sufficient attention has not yet been paid to the facility with 
which all the parts of a solid mass are shaken. The shock pro- 
duced by the head of a pin, at one of the ends of a long beam, 
makes all its fibres vibrate, and is distinctly transmitted to the 
other end. The movement of a carriage on the street, shakes 
the largest buildings, and is communicated across considerable 
