74 
to a point about 150 feet below the edge of the crater. We now saw 
repeated the conditions of 1890 with this exception, that the building-up 
process had not reached the summit. From a secure position we could 
look down upon the molten lava and observe all the phenomena of the 
immature eruptions. 
In 1896 the rising column of laya had once more forced a way for 
itself through the mountain’s base. Again the crater was a dark, roaring 
cavern. But this time the vent was in the direction of the observatory 
to the west of the summit. The liquid lava had covered many acres, de- 
stroying a part of Cook’s carriage road, and piling up a new hill a hundred 
feet high. A few inches beneath the surface this hill was still red hot, 
while from its summit two or three streams of live lava flowed sluggishly 
down its side. 
In about five years Vesuvius had passed through one constructive 
cycle. These must succeed each other until the walls of the crater have 
sufficient resistance to allow the accumulation of an explosive energy. 
Then comes the short destructive period during which the retaining walls 
are seamed and shattered. In general the number of elementary cycles 
between great paroxysms will be in direct proportion to the work of res- 
toration necessary, and this in turn will depend directly on the violence 
of the eruption immediately preceding. 
X-Ray TRANSPARENCY. By ArtTHUR L. FOoLEy. 
[Abstract.] 
Many experiments have been made to determine, and many tables 
given to show, the relative transparency of bodies to the X Rays. No 
two have been in agreement. The varied results cannot be attributed to 
uncertain methods or experimental errors, or, indeed, to the size, shape 
and general construction of the different tubes used. The degree of the 
vacuum seems to be the chief factor. 
Two of the tubes used in this investigation were of the usual type— 
non-adjustable vacuum. At first they increased in efficiency, then de- 
creased, and finally almost entirely lost their power of affecting a fluoro- 
scope or photographic plate. At first the rays possessed little penetrating 
