STUDY OF STRUCTURE OF FULGURITES 675 



nearly circular, with maximum and minimum axes as 4 : 3 ; in 

 another it is still more elliptical. This difference, common in 

 sand-fulgurites, has been attributed to distortion of the tube by 

 pressure of the surrounding sand upon the fulgurite while still 

 plastic. In one cross-section, one side of the tube appears 

 partly crushed together, with coalescence of opposite parts of 

 the wall into a blebby mass of coarse bubbles and partial oblit- 

 eration of the lumen. The characteristics of such a fulgurite 

 seem therefore to be naturally divided into those developed 

 during the sudden dilatation of the tube, and those which may 

 have ensued during its quick compression and in places partial 

 collapse. 



In regard to the distribution of the bubbles or vesicles, three 

 vaguely marked bands may be distinguished :^ a marginal tract, 

 next the lumen, comparatively free from vesicles and clear: a 

 middle portion of the wall comprising most of the larger 

 vesicles ; and the gathering of dark swarms of the more minute 

 vesicles toward the outer margin. 



The last (partly shown in Fig. 2) vastly predominate in 

 number over the larger vesicles, are almost universally spherical, 

 vary greatly in diameter down to i//. or less, and compose the 

 dark clouds, seen under low magnifying power, on inner side of 

 sand-grains adhering to outer side of the wall. Others are also 

 dispersed more irregularly in lines and bands through patches of 

 the glass (best shown under magnifying power of at least 300 

 times). A careful search was made among these dark bubbles, 

 particularly the most minute, by means of a tenth-inch objective 

 of good definition, for traces of enclosed water, but no liquid 

 could be distinguished. From this I conclude that any watery 

 vapor, derived from moisture present in the sand, has been mostly 

 expelled in the explosions, and also that this has probably had 

 far less to do, in expansion of the lumen and formation and 

 compression of bubbles, than the elastic force evolved by sudden 

 heating of the large volume of air occupying the interstices of the 



' In a fulgurite from Milton, Florida, no definite order of arrangement occurred. 

 (Merrill, loc cit., p. 90). 



