Nitrogen, is it taken into the blood from the air, 40 Nitrogen, will it burn, 44 Nitrogen, what becomes of that which is inhaled, 46 Nitrogen, where does it find a fresh supply of oxygen, 47 Nitrous oxide, why does it excite the system, 38 Non-conductors of heat, what substances are, 111 Nostrils, why are they directed downwards, 994 LESSON LII. Nose, why is it placed over the mouth, 995 Nutmegs, where are they produced, 1232 Ocean, how is it heated, 233 Oils and fats, what are vegetable, 1204 Oils and fats, animal, why are they found most abundantly in cold climates, 1205 Oils and fats, vegetable, why are they found most abundantly in hot countries, 1206 Opium, what is it, 1258 Ostriches, why have they small wings, 1031 Ostriches, why are their feathers soft and downy, 1032 Otters, seals, &c., why have they web-feet, 1062 Owls, why does their screeching denote change of weather, 1111 Owls, why does their moping foretell death, 1112 Oxen, sheep, deer, &c., why do they ruminate, 1088 Oxen, and other quadrupeds, why have they a tough elastic ligament in their necks, 1138 Oxygen, why is it necessary to life, 5 Oxygen, what is its union with carbon called, 7 LESSON LIII. Oxygen, what is it, 25 Oxygen of the air, why does it not take fire, 28 Oxygen, why do we know that it will not burn of itself, 29 Oxygen, why do we know that it is necessary to our existence, 30 Oxygen, where is it found, 31 Oxygen, why is it mixed with nitrogen in the air, 32 Oxygen of water, why does it not support fire, 307 Oxygen, in what way does man use it, 757 Ozone, what is it, 630 Ozone, why do we know that electricity produces it, 631 Ozone, what are the properties of, 632 Paleness, what is the cause of, 914 Palms, what are their characteristics, 1221 Paper held over a candle-flame, why does it become scorched, 244 Paper held below a candle-flame, why does it scarcely become warm, 245 Paper, why does it more readily ignite than wood, 294 Paper, why, if it is laid flat upon the fire, will it "char" before it ignites, 304 LESSON LIV. Paper on a fire, why will it ignite when you send a puff of air to it, 305 Parachutes, why do they fall gradually to the ground, 834 Parrots, &c., why have they crooked bills, 1047 Parrots, why can they move their upper as well as their lower beak, 1048 Particles of matter, why do they draw near each other, 776 Particles of matter, why will they attach themselves to sealing-wax excited by friction, 779 Pea and pin, why do they rotate upon a jet of air blown through a tobacco-pipe, 845 Peg-top, why does it make less noise than a humming-top, 811 Peg-top, why does it sometimes hum, and at other times not, 812 Pelican, why has it a large pouch under its bill, 1093 Pepper, where is it produced, 1229 Perspiration, why does it cool the body, 173