"I will do so with pleasure. The increased atomic motion in the heated body throws the atoms farther apart, as we have already learned, and by this increase of distance their attraction is diminished. If the earth were twice its present distance from the sun, their attraction for each other would be four times less than it now is; if its distance were three times as great, their attraction for each other would be nine times less. The attraction of gravitation diminishes in proportion as the square of the distance through which it must act increases. Perhaps cohesive attraction diminishes according to the same law, though the spaces are so small that this cannot be demonstrated, but it is certainly weakened by the expansion of bodies through the agency of heat." Here Peter raised his hand. "What will you say, Peter?" "Do not men heat and burn bricks, not to soften them, but to harden them?" "That is true," said Mr. Wilton; "but in this there is a process of drying as well as of heating, and the hardening is due chiefly to the complete drying by the intense heat. Too great heat will melt bricks while in the process of burning. I once heard a brick-burner say that he could melt the brick around the arches in his kiln in half an hour, if he pleased to put in fuel and let the fire burn. Indeed, almost every known solid substance has been fused by heat. Whether carbon has ever been melted is an unsettled question." "I would like to inquire," said Samuel, "why water will not burn. Is it because it evaporates before it reaches a sufficiently high temperature?" "This is a little aside from our subject, but the incombustibility of water is a provision of the Creator so very important that we will stop to notice it. I think, however, that by a little thought you yourself can answer the question. Tell me again what combustion is." "Combustion is commonly the combining of oxygen with some other substance called a combustible. The rusting of iron and the decay of organic bodies are forms of slow combustion." "Now tell us the composition of water." "Water is composed of oxygen and hydrogen--eight parts of oxygen to one of hydrogen, by weight, or two parts of hydrogen to one of oxygen, by measure." "How is water formed from these two gases? Are they mixed together as oxygen and nitrogen are mingled in the air, or are they chemically united?" "They are chemically united: they are burned together. When hydrogen burns, the product is water." "Water is then a _product_ of _combustion_. Can you not now tell why water is incombustible?" "I think I now see the reason. The oxygen, being itself the supporter of combustion, will not burn, and the hydrogen has been already once burned in the formation of water."