DALY. — THE NATURE OF VOLCANIC ACTION. 95 



Calculation further shows that if the compression of a considerable 

 volume of gas be isothermal, other conditions being as above assumed 

 for the adiabatic compression, the heat produced is of the same large 

 order of magnitude. 



As the lower part of the plug is fused, the liquid sinks through the 

 gas-rich part of the magma column, so that the fluxing gas is always 

 in immediate contact with the solid rock. Since the solid plug retains 

 a relatively high temperature inherited from the last active period, 

 and since the vertical axis of the plug is the hottest part of the vol- 

 canic cone at all levels above the top of the lava column, it is clear 

 that fluxing will be most rapid along the axis. 



Again, a local development of heat is to be expected as the re-fused 

 rock, which had been largely freed of gas in the last active period, 

 begins to absorb the gases collecting in the conduit. Nothing is known 

 as to the solution heat of any j uvenile gas as it is absorbed in a nat- 

 ural magma. In each case it is practically certain to be positive and 

 it may be important in amount. The data for the same gases when 

 dissolved in water have some value in the way of analogy. The follow- 

 ing table is taken from Thomsen's Thermochemistry: 



"When hydrogen dissolves in water, heat to the amount of about 800 

 cals. per gram of the gas is evolved. *2 



In this whole problem it must be remembered that hydrogen forms 

 a relatively large part of juvenile gas-mixtures. This gas has the high- 

 est specific heat of all substances yet measured, and its heat of solution 

 in water is also very high. Its efficiency in melting a volcanic plug 

 may perhaps be greater than that of the other gases and vapors put 

 together. 



In view of all the conditions, it seems correct to hold that the ac- 

 cumulation of gas beneath a solid volcanic plug develops a special kind 

 of local furnace. The energy here transformed into heat is both pt)ten- 

 tial and mechanical. In part, it is heat of solution ; in large part, it 



*' G. N. Lewis, verbal communication. 



