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Show [ 206 ] Firfl in one point the fefrcring wound confin'd Mines unperceived beneath the fhrivcl' d rind; Then climbs the branches with increafing fhcngth, 53 5 Spreads as they fpread~ and lengthens with their length ; -Thus the flight wound in graved on glafs unneal 'd Runs in white lines along the lucid field; Crack follows crack, to laws elaftic jufl:, And the frail fabric fhivers into duft. On glafs unntal'd. I. 537. The glafs makers occafiona1ly make what they call proifr which are cooled hafl:ily, whereas the other glafs veffels are remov.ed from warmer uvens to cooler ones, ami fuffered to cool by flow degrees, which is called annealing., or nealing them. If an unnealed glafs be fcratched by even a grain of fand falling - into it, it will fecm to confider of it for fome time, or even a clay, and will then crack into a thouf:md pieces. The fame happens to a fmooth-furfaced lead-ore in Derbyfhire, th~ workmen having cleared a large face of it fcratch it with picks, and in a few hour~ many tons of it crack to pieces and fall, with a kind of explofion. Whitehudl's Theory of the Earth. Glafs dropped into cold water, called Prince Rupert's drops, explode when a fmall part of their tails are broken oft~ more fuddenly indeed, but probably from the fame caufe. Are the internal particles of thefe ela!l:ic bodies kept fo far from each other by the external cru{l that they are nearly in a ffate of repulfi.on into which fl:ate they are thrown by their vibrations from any violence applied? Or, like elafl:ic balls in certain proportions fufpended in contact wi th each other, can motion once begun be increafed by their elafl:icity, till the whole explodes? And c:~n this power be applied to any mechanical purpofts? |