Trees, why have poplars comparatively few leaves, 1275 Trees, why had the mammoth comparatively few leaves, 1276 Trees, why have oaks an abundance of leaves, 1277 Trees, why are their trunks generally round, 1278 Twilight, what is the cause of, 482 Vacuum, what is a, 649 Vacuum, is it possible to obtain a perfect, 650 Vegetables, have they any heat, 90 Vegetable structures, of what do they consist, 1151 Vegetables, why do farmers sow different crops in rotation, 1164 Vegetables, why are the hearts of cabbages, &c., pale yellow, 1182 Vegetable productions, why are they so widely diffused, 1189 Vegetable eaters, why do they feed so continually, 1091 Vegetation, geographical distribution of, 1208 Vegetation, what are the characteristics of tropical, 1218 Vegetation, what are the changes in on quitting the tropics, 1232 LESSON LXXIV. Vegetation, what are the characteristics of, upon approaching the polar zones, 1234 Vegetation, what are the characteristics of mountain, 1238 Vines, where are their favourable climates, 1233 Walking, why does it make us warmer, 26 Water, what becomes of that formed by combustion, 58 Water, is it a good or bad conductor of heat, 151 Water, why does it extinguish fire, 306 Water, why, when a blacksmith thrusts a hot iron into a tank, do we perceive a peculiar smell, 309 Water, why does it freeze, 324 Water, why does it become steam, 326 Water, how many degrees of latent heat are hidden in its several states, 327 Water, why does it expand when freezing, while bodies generally contract with cold and expand with heat, 341 Water, why does it never freeze to a great depth, 342 Water, how much deeper is it than it appears to be, 480 LESSON LXXV. Water, why has the exceptional law, by which it expands when freezing, been ordained, 343 Water, why can we seldom succeed in the first attempt to touch anything lying at the bottom of, 481 Water, what causes it to flow from a pump, 651 Water, why does it run through a syphon, 655 Water, what is it, 748 Water, why does it become solid when it freezes, 751 Water, why does it boil, 753 Water, what proportion of the earth's surface is covered by it, 554 Water, why does it dissolve substances, 758 Water, why does hot dissolve more readily than cold, 759 Water, why is it sometimes hard, 760 Water, why is rain-water soft, 761 Water, why is it difficult to wash in hard, 763 Waters, why are some impregnated with mineral matters, 767 Water, why does stagnant become putrid, 769 Water, is there danger attending drinking it on account of animalculæ, 770 Water, what are the means by which it may be purified, 771 LESSON LXXVI. Water, what is the pressure of, 775 Water, why will a drop upon the blade of a knife leave a dark spot, 777