z88i] 
( 607 ) 
ANALYSES OF BOOKS. 
Volcanoes : What they are and What they Teach. By John W. 
Judd, F.R.S., Professor of Geology in the Royal School of 
Mines. London : C. Kegan Paul and Co. 
We have here a work intended both to teach and to unteach. 
The author begins by showing the erroneous nature of ordinary 
school manuals as regards this subjedt. A volcano is not neces- 
sarily a mountain, but rather a hole in the earth’s crust. When 
mountains do exist at the centres of such phenomena, they are 
formed by the matter expelled by the underground forces, — a 
mere consequence, and not a cause of what is in process. The 
adtion of a volcano bears no relation to burning; the supposed 
smoke is watery vapour, and the imaginary “flames” are merely 
the glow of melting rocks refledted from the clouds. The nature 
of volcanic adtion the author thinks may be best studied not in 
the violent outbreaks of such volcanoes as Vesuvius or Etna, but 
in the continuous and gentler adtion of Stromboli. Here it may 
be seen that the conditions on which the volcanic phenomena 
depend are the existence of apertures leading down from the 
surface to the interior of the earth, the existence of strongly 
heated matter below the surface, and the presence of quantities 
of pent up water underground, which is converted into steam 
and occasions the explosive outbursts which continually recur. 
Sulphuretted hydrogen and sulphurous acid are, indeed, emitted 
in quantities along with the steam, but the notion that the com- 
bustion of sulphur is the source of the heat is without foundation. 
Between the regular and mild adtion of Stromboli and the most 
dreadful outbreaks of Vesuvius the difference is simply one of 
degree, and a series of intermediate stages may be traced out. 
The lightning which accompanies the more violent eruptions is 
due to the fridtion produced by the escape of steam under high 
pressure from narrow orifies. 
The adtion of volcanoes teaches us some highly interesting 
lessons concerning the earth’s interior. We find that here there 
must be deposited matter of much higher specific gravity than 
that which forms the surface of the planet. The specific gravity 
of the entire globe is calculated by astronomical methods as 
being five and a half times as great as that of an equal bulk of 
water, whilst the materials of the earth’s crust range only from 
two and a third to three times the weight of water. From this 
consideration alone we are forced to the conclusion that the 
deeper regions of the earth must be of much greater density. 
This conclusion is supported by an examination of the matter 
