780 ASTONISHIKG PROPERTIES OF GLASS. 
the easier does the glasses break. One whose bottom is at least three 
bngers’ breadth in thickness, flies with as much ease as the thinnest 
glass. Some of these vessels have been tried with the strokes of a 
mallet sufficient to drive a nail into wood tolerably hard, and have 
held good without breaking. They have also resisted the shock of 
several heavy bodies, let fall into their cavities from the height of 
two or three feet ; as, musket balls, pieces of iron or other metals, 
pyrites, jasper, wood, bone, &c. But this is not surprising, as other 
glasses of similar shape and size will do the same ; but the wonder 
is, that taking a shiver of flint, of the size of a small pea, and letting 
it fall into the glass only from the height of three inches, in about 
two seconds the glass flics, and sometimes at the very moment of the 
shock ; nay, a bit of flint no larger than a grain, dropped into 
several glasses successively, though it did not immediately break them, 
yet when set by, they all flew in less than three quarters of an hour. 
Some other bodies produce the same effect, as sapphire, diamond, 
porcelain, hard-tempered steel, marble bowls, and pearls. These 
experiments were made before the Royal Society, and the effects 
were the same whether the glasses were held in the hand, put in w'ater, 
filled with water, or rested on a pillar. The glasses also broke, upon 
rubbing their bottoms slightly with the finger, within half an hour 
after rubbing: but when made uniformly very thin, they did not 
break. Hollow' cups made of green-bottle glass, three inches thick at 
bottom, w'ere instantly broken by a shiver of flint weighing about 
two grains, though they had resisted the shock of a musket-ball from 
the height of three feet. 
2. Various but unsatisfactory reasons have been assigned for these 
phenomena, by Euler and others. The effects are evidently occa- 
sioned by putting in motion some subtile fluid, from which the sub- 
stance of the glass is filled ; and the motions of this fluid, when once 
excited in a particular part of the glass, are soon propagated through 
the whole or greatest part of it, and thus the cohesive power becomes 
at last too weak to resist them. There can be little doubt that this 
fluid is that of electricity. It is known to exist in glass in a very 
great quantity; and to be capable of breaking glasses even when an- 
nealed with the greatest care, if put into too violent a motion. Probably 
the cooling of the glass hastily may make it more electric than is con- 
sistent with its cohesive power, so that it is broken by the least in- 
crease of motion in the electric fluid by friction or otherwise. This is 
evidently the case when it is broken by rubbing with the finger ; but 
why it should also break by the mere contact of flint, and other bodies 
above mentioned, bas not yet been satisfactorily accounted for. 
3. A most remarkable phenomenon is produced in glass tubes placed 
in certain circumstances. When these are laid before a fire in an 
horizontal position, having their extremities properly supported, they 
acquire a rotatory motion round their axes, and even a progressive 
motion towards the fire. When the progressive motion towards the 
fire is stopped by any obstacle, their rotation still continues. When' 
the tubes are placed in an upright posture, leaning to the right hand, 
the motion will be from E. to w. but if they lean to the left hand, 
their motion will be from w. to E., and the nearer they are placed to 
