32. Annealing of Glass.—Glass is always annealed. If this were not done our glass vessels and windows would be exceedingly brittle, and would therefore be constantly breaking. Articles made of glass are annealed by being passed very slowly indeed through a long oven which is very hot at one end, the heat gradually lessening toward the other end.
Fig. 3
33. Prince Rupert's Drops.—We have a striking example of brittleness induced by sudden cooling in what are called Prince Rupert's drops. These are made by dropping melted green glass into cold water, and they are of the shape represented in Fig. 3. If you break off ever so small a bit of the point of one of these drops, the whole will at once shiver to pieces. That is, the sudden arrangement of the particles is so slight and unnatural that the disturbance of the arrangement in a small part suffices to destroy the arrangement of the whole, very much as a row of bricks falls over from the fall of the first in the row. Mr. Farraday says that these drops were not, as is commonly supposed, invented by Prince Rupert, but were first brought to England by him in 1660. They excited much curiosity at that time, and were considered "a kind of miracle in nature." But you see that this, like many other wonders, receives with a little thought an easy explanation.
34. Malleability and Ductility.—Those metals which can be hammered into thin plates are called malleable. Gold furnishes us with the best illustration of this property. Silver, copper, and tin are quite malleable. Most of the other metals are very little so, and some of them are not at all, breaking at the first blow. A substance is said to be ductile when it can be drawn out into wire. The principal metals that have this quality are platinum, silver, iron, copper, and gold, and in the order in which I have named them. Melted glass is very ductile. It can be drawn out in a very fine thread, and when this thread is cut and arranged in branches it resembles beautiful white hair. In hammering metals into plates, or drawing them into wire, there is a considerable change of relative position in the particles, similar to that which we have in fluids, though nothing like as free. In this change of position those particles that do remain in close neighborhood have a remarkable tenacity or attraction, preventing their separation. In welding two pieces of iron, which is done by the blacksmith by hammering them together when red-hot, there must be enough movement among the particles to have those of one piece mingle somewhat with those of the other.
35. Compressibility.—Porous substances can be considerably compressed. Force applied to them can bring their particles nearer together, making them to fill up in part their pores. The most familiar example you have of this is in sponge. The more porous wood is the more can it be compressed. But even such dense substances as the metals can be compressed in some degree; that is, the interstices between their particles can be made smaller. Medals and coins have their figures and letters stamped upon them by pressure, just as impressions are made upon melted sealing-wax. The heavy and quick pressure required to do this actually compresses the whole piece of the hard metal, putting all the particles nearer together, so that it occupies less space than it did before it was stamped.
36. Incompressibility of Liquids.—We should suppose, from the freeness with which the particles of liquids move among each other, and from the spaces (§ 22) which exist among them, that these substances could be easily compressed. But it is not so. The heaviest pressure is required to compress them even in a slight degree. Water can be compressed so very little that practically it is regarded as incompressible.
37. Influence of Heat on the Bulk of Liquids.—Although the interstices between the particles of liquids can not be varied by mechanical pressure, they can be by variations of temperature. Liquids are dilated or expanded by heat; that is, their particles are put farther apart. They are contracted or compressed by cold; that is, their particles are brought nearer together by the abstraction of heat. The most familiar example that we have is in the thermometer. The mercury rises in the tube when the heat increases the interstices between its particles; and it falls when the loss of heat allows the particles to come near together. The same effects are seen when alcohol is used in the thermometer, as is done in the arctic regions, because mercury may freeze there. A thermometer with water in it would answer if we wished only to measure temperatures between the freezing point and the boiling point of water. The expansive influence of heat will be particularly treated of hereafter.
38. Compressibility of Aeriform Substances.—Aeriform bodies are more compressible than any other substances, showing that in their ordinary condition there is a great deal of space among their particles. While they are thus unlike liquids in compressibility, they are affected by heat in the same way that liquids are.