Kenry Woochvard — On Volcanos. 353 



down the pipe ; therefore, the lower stratum becomes exceedingly- 

 heated, and is able at last to overcome the great pressure, and to rise 

 into steam, carrying up with it upwards of fifty tons of water with 

 an eruption 100 feet in height. 



In order to ascertain at what depth steam was formed in the tube 

 of the G-eyser, Professor Bunsen suspended stones at the ends of 

 cords (Fig. 11, a, b, c), of various lengths, in the tube. When an 

 explosion took place the stone at c was found to be blown out, whilst 

 the stones a, b, remained undisturbed. He thus learnt that it was 

 between A and B that steam was formed ; in fact, just at that point 

 in the tube where the observed temperature approaches most nearly 

 the known boiling point for water under the pressure of a column of 

 water of about 40 feet. 



Eeferring the Geyser-action to the crater of a volcano, where steam 

 in a white-hot state under enormous pressure must exist, we can 

 readily see the explanation of the rhythmical explosions which some 

 volcanos constantly keep up. 



So long ago as 1825, Mr. Scrope arrived at the conclusion that the 

 mobility of the solid component particles of liquid lava was not due 

 to the mass being in a state of molecular fusion — in which condition 

 it never occurs subaerially — but to the presence of an interstitial 

 fluid disseminated through the mass, and that this fluid was water in 

 a highly comminuted condition. This conclusion he seems to have 

 arrived at from observing that the incandescent lava at the moment 

 of its exposure, and in the act of consolidation, always gave off 

 abundance of steam. 



"Water," says Mr. Scrope, '-'we know is converted into vapour 

 only at temperatures increased in proportion to the increased pres- 

 sure to which it may be subjected; and when altogether hindered 

 from communication with the atmosphere, as in a Papin's digester or 

 other closed vessel, may be made red-hot, without expanding into 

 vapour. 



" The moment, however, that the opening is made in the inclosing 

 vessel (reducing the pressure to that of the atmosphere only), it 

 flashes instantly into steam with explosive violence. The same 

 effect of course must take place in an imperfect liquid or paste com- 

 posed of water, and any solid matter in mechanical suspension or 

 mixture, such as flour, clay, sand, or any other granular substance." 

 (Scrope, "Volcanos," p. 39.) 



The theories as to the source of volcanic heat are many and various ; 

 but the one which may be said to claim the largest share of support 

 is that which attributes the phenomena of volcanos and earthquakes 

 to the reaction of the interior of our planet upon its uppermost strata. 



A volcano appears always to keep up a permanent connexion be- 

 tween the interior of the earth and the atmosphere, or the sea, either 

 directly or indirectly ; sea- water being doubtless the source of many 

 mineral ingredients in volcanic products. 



Mr. Hopkins, nearly thirty years ago, and subsequently Arch- 

 deacon Pratt and Sir William Thomson, have condemned as un- 

 tenable and contrary to the known laws of precession and nutation 



VOL. VIU. — NO. LXXXVI. 23 



