As the specific gravity of the crystallized drop is nearly the same as that of the annealed drop, the cavities must be produced by the contraction which the internal part experiences in cooling, for the sudden induration of the outer layer prevents the contraction from taking place in any other way. The manner, too, in which the cavities disappear, is a complete proof that they contain no air, and hence we may consider their magnitude, which in creases with the size of the drop, as a 'Leasure of the con traction which the glass undergoes in its transition from the temperature at which it melts, to the ordinary tem perature of the atmosphere. See EXPANSION.
1 am informed by Dr Hope, that he has obtained un annealed drops of crown glass, in which there were no vacuities, and that they all burst spontaneously in the course of a few months." Phil. Trans. 1815, p. 1.
The unannealed glass cups, which we have mention ed under ANNEALING, are represented in Fig. 5. Plate CCLXXIV. The lower end 13 is made very thick, and the bodies, such as a musket ball or a fragment of flint, are dropped into the mouth of it at A. The stroke of the ball upon the thick bottom will produce no effect, while the blow of the small fragment of flint will burst the cup with great violence. The following are the dimensions of the cup represented in Fig. 5 VI inches ; width at top I ; width at bottom I ; thickness of glass at ; greatest thickness of glass at bottom The bursting of these cups is effected, when they are even three inches thick at the bottom. In an experiment made by Dr Littleton upon a cup of this magnitude, it resisted a blow from a musket ball let fall from a height of nearly three feet, while it was instantly broken by a shiver of flint weighing only two grains. An account of numerous experiments made with these cups will be found in the Philosophical Transactions for 1745, vol. xliii. No. 477, p. 505.
The Right Hon. Sir Robert Moray discovered that hollow balls made of unannealed glass, with a small hole in them, would be burst in pieces by the heat of the hand alone, by stopping up merely the small hole with the finger. This obviously arose from the pressure of the expanded air on the interior of the ball.
About the year 1740, when Mr C. Orme, of Ashby de is Zouch, was drying the glass tubes for his diagonal barometers, he observed that they had not only a rotatory motion about their axis, but also a progressive motion towards the fire. These tubes were about four feet long, and half an inch thick, and when placed about 6 or 8 inches from the fire, they moved rr not only progressively, but about their axis along the side wall against which they leant, and along the front wall of the chimney, which made an obtuse angle with the other, so that they seemed to move up hill, and against their weight." The Rev. Gran
ville Wheler, to whom Mr Orme shewed these experi ments, repeated them with great care, and found that the experiment succeeded best with a moderate fire, and when the tubes were about 20 or 22 inches long, and about one tenth of an inch in diameter, when they had in each end a pretty strong pin fixed in a cork as an axis, and when they were supported on other glass tubes of nearly the same diameter. When the progressive motion of the tubes was stopped by an obstacle, they still revolved about their axis. When the tubes were placed horizontally on a large frag ment of plate glass, instead of advancing towards the fire as formerly, they moved from the fire, and about their axis in a direction contrary to what they had clone before. In this ease, as formerly, the tubes receded from the fire, even when the plate of glass was a little inclined. Mr Wilder very ingeniously explains these phenomena by the expansion of the parts of the tube nearest the fire, which, by placing the glass at a greater distance from the centre of motion, destroys its equilibrium. The heavy side of the tube therefore descends, and a fresh part of it being exposed to the fire, expands and descends as for merly. A writer in a modern dictionary opposes this ex planation, un the ground that " the fundamental principle on which it proceeds is false, for though lire, indeed, will make bodies expand, it does not increase them in weight, and therefore the sides of the tube, though one of them is expanded by the Ire, must still remain in equilibria and hence we must conclude that the causes of these pile " nomena remain yet to be discovered." In this extraor dinary reasoning, the author has overlooked the funda mental truth in mechanics, that the force with which any quantity of matter tends to turn round a fulcrum, is pro portional to the sum of all the products of each particle of matter multiplied by its distance from the centre of motion. In the case of the glass tube, the number of par ticles remains the same, and the distance of all of them from the centte of motion is increased. Hence the sum of the products is increased, and consequently the equilibrium destroyed.