VOL. LXX.] PHILOSOPHICAL TRANSACTIONS. 623 



Vesuvius; the one was 22i feet long, 13^ feet broad, and 10 feet high; tlie 

 other, 1 li feet high, and 72 feet in circumference. 



They found also many fragments of those volcanic bombs that were said to 

 have burst in the air; and some entire, having fallen to the ground without 

 bursting. The fresh red-hot and liquid lava having been thrown up with num- 

 berless fragments of ancient lavas, the latter were often closely enveloped by the 

 former; and probably when such fragments of lava were porous and full of air 

 bubbles, as is often the case, the extreme outward heat, suddenly rarifying the 

 confined air, caused an explosion. When these fragments were of a more com- 

 pact lava they did not explode, but were simply inclosed by the fresh lava, and 

 acquired a spherical form by whirling in the air, or rolling down the steep sides 

 of the volcano. The shell or outward coat of the bombs that burst, was always 

 composed of fresh lava, in which many splinters of the more ancient lava that 

 had been inclosed are seen sticking. 



The phenomenon of the natural spun-glass, which fell at Ottaiano with the 

 ashes on the 5th of August, was likewise clearly explained here. It has already 

 been mentioned, that the lava thrown up by this eruption was in general more 

 perfectly vitrified than that of any former eruption, which appeared plainly on a 

 nearer examination of the fragments of fresh lava, the pores of which were 

 generally found full of a pure vitrification, and the scoriae themselves, on a 

 close examination with a magnifying glass, appeared like a confused heap of 

 filaments of a foul vitrification. When a piece of the solid fresh lava had been 

 cracked in its fall without separating entirely, he always saw capillary fibres of 

 perfect glass, reaching from side to side, within the cracks. Sir Wm. explains 

 his idea of this lava by comparing it with a rich Parmesan cheese, which, when 

 broken and gently separated, spins out transparent filaments from the little cells 

 that contained the clammy liquor of which those filaments were composed. The 

 natural spun-glass therefore, that fell at Ottaiano during this eruption, as well 

 as that which fell in the Isle of Bourbon in 1766, must have been formed, most 

 probably, by the operation of such a sort of lava as has been just described, 

 cracking and separating in the air at the time of its emission from the craters of 

 the volcanoes, and by that means spinning out the pure vitrified matter from its 

 pores or cells, the wind at the same time carrying ofi^ those filaments of glass as 

 they were produced. 



Sir Wm. observed, sticking to some very large fragments of the new lava, 

 which were of a close grain, some pieces of a substance, whose texture very 

 much resembled that of a true pumice stone ; and on a close examination, and 

 having separated them from the lava, he perceived that this substance had actually 

 been forced out of the minute pores of the solid stone itself, and was a collection 

 of fine vitreous fibres or filaments, confounded together at the time of their 



