48 BOLOGNA PHIAL. to the process I have just described, is said to be unannealed. One specimen of this kind is called the Bologna phial. It is shaped like common phials, and may be made of any kind of glass; but the bottom of it must be a great deal thicker than the upper part, and when made it must be cooled suddenly in the air. Some of these phials have been struck by a wooden mallet, with force sufficient to drive a nail into most kinds of wood, and the glass remained uninjured; yet they broke readily when a small sharp piece of flint, weighing only two grains, was dropped gently into them even a grain of sand has produced this effect, causing the thick bottom of the phial to crack all round and drop off. When the falling substance has been very hard and sharp-a cut diamond, for instance-it has been seen to pass through the thick bottom of the glass, apparently meeting with as little resistance as would be offered by a cobweb. This seems a marvellous story, but there is no disputing well-authenticated facts. There RUPERT'S DROPS. 49 are various explanations of the phenomenon. Some ascribe it to a different arrangement of the particles of glass, in consequence of its hasty cooling; others, to its being rendered more electric by that means; but I believe all agree that the disproportioned thickness of the bottom is an indispensable circumstance in the experiment; and that the thicker it is, the more easily the phial will be broken by the minute and sudden shock it receives. If I knew where to procure a Bologna phial, I should be glad to show you the experiment. There seemed more probability of obtaining another kind of unannealed glass, called Rupert's drops. After many fruitless inquiries, and some unsuccessful attempts to make such drops myself, I was on the point of giving it up altogether, when a shopkeeper very obligingly sent me the real thing, which I have now the pleasure of showing you. These drops are small, solid pieces of common green glass, which have been suffered to fall, while red-hot, into cold water, where they D 50 RUPERT'S DROPS. took the form you see, that of roundish lumps, lengthening into a sort of tail. The thick, round part will bear a hard blow; but if one of you will grasp it in your hand, while I break off only the tip of the tail, the whole drop will burst into minute fragments. I have heard, that if one of these drops is put into a phial or tall glass filled with water, and the end of it is broken off with a pair of pincers, the bulb will fly in pieces with such force as to break the vessel in which it is contained.* It is said, that the stoutest wine or beer bottle would not be strong enough to withstand the shock. We will not attempt this experiment, as I am afraid some particles of glass may be thrown amongst you. You may believe the fact, if you please, on the authority of Dr. Lardner. But if any of you will hold a Rupert's drop firmly in your closed hand, while I break off the tip of the tail, you may do it very safely; and observe how completely the particles of glass will be separated by that slight shock. *Lardner, 173, 175. REFLECTING POWER OF GLASS. 51 It would be useless to enumerate all the ingredients employed by glass-makers, because you would not recollect the names of them. If you compare these two specimens, the one of flint-glass, and the other of plate-glass, you will perceive an evident difference in their colour. Place the edges of the two pieces side by side, and you may observe it distinctly: the plate-glass is considerably darker than the other. This is produced by the addition of a metal called manganese, which also increases the reflecting power of the glass: this is an advantage, as plate-glass is generally used for looking-glasses.* When you are passing along the street, you may observe a shop which has windows of plateglass, and compare its power of reflecting objects with that of common crown-glass in the windows of the next shop. You will at once perceive that there is a great difference between them. But to make plate-glass completely answer the purpose of a mirror, we must call in * Ibid, and Gray's Op. Chem. 559 52 SILVERING LOOKING-GLASSES. the assistance of metals, which are applied in so simple a way that I think you can easily understand it. Here is the effect you see, in the silvery appearance on the back of this piece of glass. I will tell you how it is produced. A smooth, thick slab of wood or stone, inclosed in a wooden frame, is placed on a pivot, so that either end may be raised or lowered at pleasure. When it is used, the slab must be placed in a level position, and covered with paper. A sheet of very thin tin-foil is then laid upon the paper; and as much quicksilver is steadily poured over it as will remain on its flat surface. Now comes the difficulty-the nicety of the operation, which consists in sliding the plate of glass to be silvered so dexterously into the frame, that it may just dip into the surface of the quicksilver as it glides along, without once touching the tin-foil that lies beneath it, When this is happily accomplished, the plate is suffered to drop gently down by its own weight. Some of the quicksilver is thus squeezed out from between the glass and the tin-foil, but |