102 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
the vapor of water which is the main impelling agent in volcanic work 
gathers into bubbles before the ejection takes place. If this be the 
case, we have yet another likeness of the actions which go on in the 
crater pipes and in the springs as before described. I am fortunately 
able to give the results of some observations on one of the eruptions 
of Vesuvius which throw much light on this point. 
In 1879 while Vesuvius was in a state of slight activity, I managed 
to see a little further into the throat of the crater than is commonly 
possible in the time of outbreak. At this time the eruption was from 
a central cone which rose to a height of two or three hundred feet 
above the general plane of the summit. Availing myself of the chance 
afforded by a very strong gale from the northwest, I managed to gain 
the edge of the crater without danger from the fragments of lava 
which the explosions were hurling to a height of a thousand feet or 
more, the wind being strong enough to drift these projectiles to the 
leeward side of the cone. For about half an hour, despite the diffi¬ 
culties of the position, I had a chance to see what took place at the 
moment of explosion; the important facts were as follows : — the 
crater had a width of three or four hundred feet, its steeply sloping- 
sides terminating in a vertical pipe which had a diameter of from 
sixty to one hundred feet. In this pipe there was a column of white- 
hot lava, which swayed up and down apparently through a range of 
some hundred feet; when it came up to the bottom of the cup, 
expanding in the enlargement, the mass parted, giving forth in the 
manner of a geyser a rush of steam. As this steam was at first, 
because of its high temperature, quite transparent, the parting of the 
lava to give passage to this huge bubble was again and again dis¬ 
tinctly seen. The energy of the outbursts was so great that each 
time one occurred I was thrown back somewhat from the edge of the 
crater, but as my face was protected by a mask from the intense heat 
which accompanied each explosion and as there was little lateral 
movement to the fragments, I was able to note the phenomenon in 
a clear way. The fact that the steam had gathered in masses of great 
size while it was below the surface seemed to me perfectly well 
observed. In near a hundred of these explosions which took place 
during the half hour while I was able to observe them the bubble-like 
nature of the outbreaking vapors was demonstrated. There was 
evidently a constant procession of these great vesicles making their 
way up the pipe. Whenever the fluid lava expanded in the crater, the 
sudden removal of the pressure enabled them to burst. 
