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SCIENCE 



[N. S. Vol. XXVIII. No. 713 



sequently subject to the laws of hydrostatics. 

 With this understanding let us examine for 

 a moment the points of analogy and of dif- 

 ference between the two. 



In the lower portions of the geyser-tube the 

 water becomes heated by conduction above 

 100° C. Ebullition can not take place be- 

 cause of the pressure of the water above, and 

 the excess of heat represents stored energy — 

 a latent force which will manifest itself upon 

 relief of the pressure due to the above stand- 

 ing water column. This latter may be con- 

 sidered as divided into an indefinite number 

 of zones each having a critical temperature 

 depending on its position, that of the surface 

 layer being 100° C. The column of water is 

 progressively heated from below by conduc- 

 tion and convection until the water of some 

 zone attains its critical temperature; boiling 

 takes place, relieving the pressure on the 

 water just below, which, in its turn, bursts 

 into ebullition, and thus a progressive reac- 

 tionary movement is set up with a rapidly 

 increasing amplitude of vibration until most 

 of the energy latent in the superheated 

 depths is set free, completing the eruption. 

 The action is often begun by a raising of the 

 water, which, at some zone, is near its critical 

 temperature, into a position of lesser pressure, 

 when boiling will begin and the reactionary 

 process be initiated — in either case it will be 

 noted that it is the rapid diminution of pres- 

 sure by the act of ebullition which institutes 

 the vibratory process. The reader will here 

 recall that, in the bursting of steam boilers, 

 the action is also thought to be multiple, the 

 too rapid escape of steam from a broken part 

 resulting in the sudden liberation of energy 

 latent in the superheated water, thus com- 

 pleting the explosion. 



Let us now consider the action of a basaltic 

 volcano, assuming the central conduit to be 

 filled with liquid magma up to the crater. 

 The lava in the conduit below the crater will 

 be subjected to a pressure increasing pro- 

 portionately with the depth, and the water 

 and other gases occluded in the magmatic 

 material will, under such conditions of pres- 

 sure and temperature, be possessed of an 



enormous latent force of expansion. An up- 

 forcing of the lava column or a rapid in- 

 crease of temperature may, therefore, precipi- 

 tate an eruption by instituting a reactionary 

 process of gaseous expansion exactly as in the 

 case of the geyser. The greater dimensions 

 of the volcano, together with the density of 

 the magma, will render this reactionary proc- 

 ess more gradual than in the geyser; inertia 

 and momentum will prolong the vibratory 

 periods, and days instead of minutes may be 

 required to bring about the culmination. It 

 may, indeed, be questioned if, in many cases, 

 an elevation of the lava column, or an in- 

 crease in its temperature would be sufficiently 

 sudden to initiate the reactionary process, 

 but this may be brought about in another 

 way. An interesting point of divergence 

 from the geyser lies in the height of the 

 volcanic cone within which the lava may 

 rise to a considerable elevation above the 

 earth's surface. Pressure of the lava column 

 on the walls of the cone aided by explosions 

 from below and the re-fusing power of the 

 magma may fissure the cone and permit of a 

 lateral outflow. If this is sufficiently rapid 

 to considerably reduce the level of the lava, 

 the pressure on the magma below is greatly 

 diminished and gaseous expansion takes 

 place, an immense amount of vapor is set 

 free to do battle with the solid materials 

 (due to collapse consequent to the withdrawal 

 of a large quantity of lava), and a great 

 eruption is thus produced. In my opinion, 

 we need not conclude that the rapid gaseous 

 expansion extends to the greater depth of the 

 volcanic conduit and much less to the fire- 

 pocket itself, where the magma, by reason of 

 pressure, may be in a pasty or quasi-solid 

 condition, but the active expansion would be 

 limited to a zone whose depth will bear a cer- 

 tain relation to the original height of the 

 lava column and the difference of level re- 

 sulting from the outflow. The greater the 

 difierence of level the deeper will be the zona 

 of active expansion and the sum total of 

 energy released. The more rapidly the dis- 

 leveling is produced, the more violent will 

 be the explosive effects, although the total 



