130 Scientific Proceedings, Royal Dublin Society. 



One is apt, however, to attach a greater value as regards 

 quantity to volcanic vapour, from the peculiar molecular state 

 which it assumes immediately on its escape, which is probably due 

 to conversion of the steam into vapour, by the process investigated 

 by Dr. Aiken. Everyone is aware that our breath in the hottest 

 weather is converted into a white cloud when near the vapour of 

 of HC1. Nevertheless, whatever value we may put upon the 

 above calculation, we cannot do otherwise than comprehend the 

 very large amount of water that may disengage itself from the 

 igneous magma. 



In the case of a fissure whose upper limits are very far be- 

 neath the surface, and suddenly extends thereto, we should expect 

 the eruption to be less violent than were the magma in closer 

 proximity ; since a large part of the energy of tension would be 

 lost in the expansion in injecting the extension of the fissure. 



Conditions which may determine an Eruption. — In a large number 

 of cases the gradual increase of tension in the confined magma may 

 go on to the crisis of eruption. But in certain cases the intervention 

 of collateral influences may anticipate such an occurrence. An in- 

 crease of upward pressure from the main volcanic source, dependent 

 upon secular cooling, tidal actkn (if such exists), or other causes, 

 may be sufficient addition to the amount of tension already exist- 

 ing to more than balance the resistance. A sudden lowering of 

 atmospheric pressure may be sufficient in some cases to render the 

 superincumbent pressure less than the tension of the igneous 

 magma. It is known that, as the rainfall is increased in the 

 season, the drainage level of a country reaches a higher line, and 

 therefore the superincumbent pressure increases; and, vice versa, the 

 superincumbent pressure diminishes during a drought, so that a 

 sudden relief of pressure may be the metaphorical last straw. 



The greater the height in a temporarily extinguished volcano 

 the greater the weight, or, in an active one, the greater the pres- 

 sure of the superincumbent column of lava above the drainage 

 level. 1 We might therefore say, that as a permanent volcano in- 

 creases in height its eruptions will diminish in frequency to increase 



1 Let us compare the height of a column of phonolite paste of 100 (?) meters of Monte 

 Nuovo with the column of heavier doleritic paste of Etna of 3300 meters, when we see 

 that this is an important factor in modifying the eruptive character of a volcano. 



