"One of our earliest lessons," said Mr. Wilson, "taught us the nature of porous bodies; we were soon able todistinguish these bodies, either by actually seeing the pores themselves, as in sponge, bread, charcoal, pumice-stone, and cane, or by watching theiraction in absorbing liquids, as in chalk, lump-sugar, and dry clay. Our experiment with the chamois-leather and mercury proved the porosity of the leather, for the little balls of mercury passed through its pores.
"Before we go any further I will show you a very pretty experiment with the air-pump. I have here a piece of thick malacca cane, pointed at one end, and hollowed out into a little cup at the other. I will place thecane in the open mouth of the receiver, with the pointed end downwards, takingcare to see that it fits air-tight. I will next pour a little mercury into the cup at the top, and then exhaust the air from the receiver.
As the receiver is exhausted, the pressure of the outer air drives the mercury downwards through the pores of the cane, and a shower of tiny drops of the metal can be seen falling from the lower end.
"In the cane and many other bodies the pores can be readily seen; they are then called sensible pores. But in some bodies the pores are too small to be detected, even with the aid of the microscope. And now, I suppose, you are wondering how we know that such bodies have pores at all. I will explain it to you. A piece of metal, say copper or steel, does not look very porous. The most powerful microscope could not detect any pores in it. But by placing it in a freezing mixture of salt and snow, it can be made to shrink up or contract with the cold.
"What does this shrinking mean? It means that the molecules are driven closer together; therefore, there must, under ordinary circumstances, be spaces between them. These spaces between the molecules are the pores. Such pores are called physical pores. Every solid substance is more or less porous.
"Liquids are porous too, although, in this case again, it is impossible to detect the pores. Can you think of any of our old experiments which prove that liquids are porous?" "Yes, sir," said Fred. "We filled a tumbler with water to the very brim, and then put in salt, a spoonful at a time, stirring it all the while. The salt disappeared, but none of the water overflowed. The salt found room by filling upthe spaces or pores between the molecules of water." "That"s the very one I was thinking of, my lad," saidMr. Wilson, "and now I suppose you will not be surprised that porosity is one of the general properties of matter- in other words, that all matter is more or less porous.
"We are now in a position to investigate another of the general properties of matter-that of compressibility.
"From what we have said it will be easy to understandthat the more porous a body is, the more compressible it must be. The most compressible bodies are gases, which are also the most porous, because their molecules are at a considerable distance from one another. It is quite possible to compress one hundred gallons of air into the hulk of a single gallon.
"Just as solids vary in porosity, so do they vary in compressibility. Some, such as wood, cork, and sponge, are very compressible; others, such as glass, have very slight compressibility; pressure will rather reduce them to powder than lessen their bulk.
"Look now at these coins. The impression on them proves the metal of which they are made to be compressible, because the impression was produced by pressing the molecules closer together.
"Liquids are the least compressible of all bodies. It has been proved that 20,000 cubic inches of water cannot be reduced, by the utmost pressure, to less bulk than 19,999 cubic inches. This slight reduction in bulk explains why in your earlier lessons you have been taught that water is incompressible, and to all practical purposes it is so.
"We will now turn our attention to the last of these general properties of matter-that of elasticity.
"It is the power which bodies have of springing back totheir former shape after they have been interfered with. It is a common property of both solids, liquids, and gases.
"Some bodies, as we have seen, show their elasticity after being squeezed; among these are such common objects as sponge, cork, and wool. Others, such as india-rubber, flannel, and cloth, require to be pulled, to show their elasticity. Others, again, only show their elasticity whenthey are bent ; among these are cane, whalebone, and steel. "Take this air-balloon and throw it on the table. It springs upwards. The balloon, in striking the table, became flattened, and the air in it was compressed. The air, however, is elastic, and immediately after being compressed it sprangback to regain its original bulk. It was the springing back of the air that caused the balloon to fly upwards.
"This elasticity of the air in the air-balloon is easy to understand, but I will now prove to you that even such hard bodies as glass are elastic.
"I have smeared the smooth, polished surface of this slab of marble with ink. Watch what happens when I drop this glass ball on it. It strikes the slab and rebounds. The ball touches the slab of course at one spot only, but when we look at the ink on the slab, we find a large circular mark. The fact is, the ball on striking the slab became flattened, and in that condition the mark was made. Its own elasticity, however, caused it to spring back at once to its original shape, and the springing back gave it the upward rebound."
