A. Because (when the moisture of the mould has been evaporated by the sun) the earthy particles come into closer contact, and the mass becomes more solid.
Q. Show the wisdom of god in this arrangement.
A. If the soil did not become crusty and hard in dry weather, the heat and drought would penetrate the soil, and kill both seeds and roots.
Q. Why is tea cooled faster in a saucer than in a cup?
A. Because evaporation is increased by increasing the surface; and as tea in a saucer presents a much larger surface to the air, its heat is more rapidly carried off by evaporation.
(The subject of “convection” will be treated of in a future chapter, and would scarcely be understood in this place.)
Q. Why is not the vapour of the sea salt?
A. Because the salt is always left behind, by the process of evaporation.
Q. Why does a white crust appear (in hot weather) upon clothes wetted by sea water?