The workman now receives from his attendant the pontil with the glass vessel attached, and after re-heating it at the furnace mouth as before, seats himself on a sort of stool provided with arms sloping forward, whereon the pontil is supported before him in a horizontal position, the glass being at the man's right hand. Thus placed he governs with his left hand the movements of the pontil by twirling it to and fro along the arms of the stool; and taking in his right hand an iron instrument, called a procello, the blades of which are connected together by an elastic bow, in the manner of a pair of Fig. 10.

sugar tongs, he enlarges or contracts the vessel in different places until it assumes the requisite form.

Any superabundance of material is cut away by the scissors while the glass is red-hot, with as much ease as they could be made to divide a piece of soft leather.

If the article is of a kind, by its size or by the com

plex nature of its form, to occupy much time in its manufacture, it must occasionally be re-heated, as its increasing rigidity makes it more difficult of manage

ment.

To insure the requisite regularity in shape and size, the workman is provided with compasses and a scale marked off in feet and inches.

The finished article is detached from the pontil in the same manner as it has previously been from the blowing tube, by wetting it at the point where it is attached, and is then dropped gently on a bed of ashes kept at the manʼs side for the purpose. It is then taken up on a pronged stick, or, when from its shape this mode is inconvenient or impracticable, the glass is made to rest during the detaching process on a wooden shovel, wherein it consequently remains when divided. In either case a boy in attendance conveys it without loss of time, and while yet exceedingly hot, to the annealing oven.

If it be required to give to the article any form or pattern which is unattainable by the simple means narrated, a mould is provided, into which the glass is placed while it is being blown, and where it receives the requisite impression with as much facility and faithful

ness as wax.

The process of annealing is one of very great importance; without it, glass would be liable to fly with the smallest change of temperature, and would break with the merest scratch or touch, or even without any apparent external cause of injury. The most reasonable theory which has been proposed in explanation of this disposition in unannealed glass is, that by its sudden cooling the external particles are forcibly contracted, while the inner substance still remains soft and expanded. The two portions thus take up positions, in relation to each other, very different to those which they would occupy if the contraction of the whole had gone forward equally and gradually. By this means a constant strain is kept up between the different parts, and

should a force be applied of a nature to rive any the smallest portion asunder, the equilibrium of resistance is deranged, and the elastic quality of the glass causes the injury to be felt strongly and suddenly, but very unequally through the whole mass.

This theory appears to receive confirmation from the well known and often repeated experiments made with the Bologna phial and with Rupert's drops. The first is a phial of ordinary shape made of any kind of glass, much thicker at its bottom than in its upper portion, and which has been suddenly cooled in the air. This phial, from its thickness, will sustain a considerable blow from any blunt instrument, or will bear uninjured the sudden concussion caused by the fall of a leaden bullet. But if any hard and angular substance, such as a minute portion of gun flint or even a grain of sand, be drop. into it, the bottom will crack all round and drop off In performing this experiment if the glass be very brittle and the substance dropt upon it be very hard and sharp- a cut diamond for instance - this has been seen to pass through the thick bottom with apparently as little resistance as would be offered by a cobweb.

The greater comparative thickness of the bottom is an indispensable quality of these Bologna phials, and the more considerable this disproportion is made, the more easily will the disruption be effected. Some of these vessels have been struck by a wooden mallet and were uninjured notwithstanding the force applied was sufficient to drive a nail into most kinds of wood; and yet the glasses broke readily when a small shiver of flint weighing only two grains was gently dropt within them. Flint being very hard, and its angles when fractured extremely sharp, its points of contact with the glass are exceedingly small, so that the effect produced by even so very minute a portion of this substance will be comparatively greater than would accompany the blow given by a much larger but softer and less angular body, and which for these reasons would

divide the shock between a greater comparative number of particles of the glass.

Another theory has been proposed in order to account for this singular property in certain forms of unannealed glass. It has been imagined that the sudden cooling of glass may occasion it to be more electric than is consistent with the cohesive attraction of its particles, and that the sudden setting in motion of the electric fluid which glass contains, may occasion throughout the substance a propagation of the motion of that fluid, which will go on accumulating within itself a force too great to be at length resisted.

This theory is by no means free from difficulties, yet it seems to derive support from a fact which was developed in the course of some experiments made before the Royal Society, in which glass vessels, the thick bottoms of which were only slightly rubbed by the finger, broke after the interval of half an hour had occurred from the time of rubbing.

Rupert's drops are small solid pieces of common green glass, which have been dropped while red-hot into cold water, and which are thus caused to take the form of rounded lumps elongated into a kind of tail. The spherical part will bear very rough treatment without injury; but if the smallest portion of the tail be broken off, the whole article instantly bursts into a countless number of fragments, so minute as to produce only a slight stinging sensation in the hand by the sudden disruption. If one of these drops is immersed in a phial or tall glass filled with water, and its end be broken off with a pair of pincers, the bulb will be rent so suddenly and with so great a force as will infallibly break the vessel wherein it is contained. The stoutest wine or beer bottle would not be strong enough to withstand the shock.

Messrs. Aikin completely destroyed this property in drops of this kind and in Bologna phials, by heating them to redness and then allowing them to cool gra

dually as in the annealing oven. Not only was their quality of bursting corrected by this treatment, but the particles of glass were made to assume a closer union among themselves, which fact was proved by the acquirement of a sensible increase in their specific gravity.

The internal form of an annealing oven has been already described. Articles newly made are placed on

the shallow trays previously mentioned, in the part of the oven most exposed to the heat of the fire, which, it will be remembered, is kindled under one end only.

Fig. 15.

Each one of these lier pans or fraiches, as it is filled, is pushed forward in the oven, towards the colder end, to make place for a fresh tray, until the articles, at length and in succession, reach the farthest extremity of the oven, whence they are taken, but little warmer than the temperature of the atmosphere.