PROPERTIES OF GLASS. 159 



force of percussion which broke the glasses, undoubtedly the 

 fracture would always take place at the very instant of the stroke; 

 but we have seen, that this did not happen sometimes till a very 

 considerable space of time had elapsed. It is evident, therefore, 

 that this efl'ect is occasioned b) the putting in rnotioa some subtile 

 fluid with which the substance of the glass is filled, and that the 

 motions of this fluid, when once excited in a particular part of the 

 glass, soon propagate themselves through the whole or greatest 

 part of it, by which means the cohesive power becomes at last too 

 weak to resist them. There can be little doubt that the fluid just 

 now mentioned is that of electricity. It is known to exist in glass in 

 very great quantity ; and it also is known to be capable of breaking 

 glasses, even when annealed with the greatest care, if put into too 

 violent a motion. Probably the cooling of glass hastily may make 

 it more electric than is consistent with its cohesive power, so that 

 it is broken by the least increase 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 the other bodies abovementioned, has 

 not yet been satisfactorily accounted for. 



A most remarkable phenomenon also 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 axis, and 

 also a progressive motion towards the fire, even when their sup. 

 ports are declining from the fire, so that the tubes will move a little 

 way up hill towards the fire. When the tubes are placed in a 

 nearly upright posture, leaning to the right hand, the motion will 

 be from east to west; but if they lean to the left hand, their mo- 

 tion will be from west to east ; and the nearer they are placed to 

 the perfectly upright posture, the less will the motion be either 

 way. If the tube is placed horizontally on a glass plane, the frag- 

 ment, for instance, of coach window-glass, instead of moving to- 

 wards the fire, it will move from it, and about its axis in a con. 

 trary direction to what it had done before; nay, it will recede 

 from the fire, and move a little up hill when the plane inclines 

 towards the fire. These experiments are recorded in the Philoso- 

 phical Transactions. They succeeded best with tubes about twenty 

 or twenty.two inches long, which had in each end pretty strong 

 pin fixed in cork for an axis. 



