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they show a ragged appearance, all the sinuosities being curvilinear 
This mode of fracture is in correspondence with the vacuolated 
structure, and, just as in the porous pumice, the vitreous volcanic 
ashes are permeated by vacuoles ; besides, everything goes to show 
that the fragmentary condition and the fresh fractures are due to a 
tension phenomenon which alfects these vitreous matters in a manner 
analogous to what is observed in the “Eupert’s drops.” 
Fig. 1. — Yitreous particles of the Ashes of Krakatoa, which fell at Batavia, 
27th August 1883 
AYe have pointed out that brown vitreous fragments are rare in 
the ashes of Krakatoa. These, however, contain skeletons of 
magnetic iron, and are de vitrified by microliths.^- It is scarcely 
necessary to add that the particles, whose form we have indicated, 
are isotropic. If under crossed nicols we sometimes see the field 
illuminated, this is due to crystals in the vitreous matter, or to 
phenomena of tension, which are sometimes observed in the neigh- 
bourhood of the bubbles. 
These details on the microstructure of the vitreous particles from 
Krakatoa can be applied with most perfect exactitude to the vol- 
canic dusts, which we have determined as such, in the deep sea 
deposits. In virtue of their bulbous structure, their dimensions, and 
their mode of projection, they are capable of being widely transported 
from the point of eruption by aerial currents. It must be admitted, 
* Just as we can divide pumice microscopically according as it is acid or 
basic, so the products of its trituration may be recognised under the microscope, 
inasmuch as the former often give colourless and more elongated particles, 
while the fragments of basic pumice have a more pronounced tint and more 
rounded pores. 
