The Reason Why. Robert Kemp Philp

       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

       Perspire, why do we, 1009

       Perspiration, how is it formed, 1010

       Perspiration, what is insensible, 1011

       Perspiration, what is sensible, 1012

       LESSON LV.

       Phosphorous, why does it look luminous, 94

       Phosphorous, does it ignite spontaneously when placed upon a hot surface, 314

       Phosphorous, why does it ignite when sprinkled with powdered charcoal, 315

       Photographic pictures, how does light produce them, 556

       Photographic rooms, why are they glazed with blue glass, 557

       Pith-tumblers, why do they always pitch on one end, 868

       Planets, how can man weigh them, 785

       Planets, how can man measure their distances, 786

       Plants, do they absorb heat, 225

       Plants, how do we know that they absorb heat, 226

       Plants, why do screens prevent frost from killing them, 400

       Plants, why, if we cut across their stems, do we see tubes arranged in parallel lines, 1159

       Plants, why do some droop and turn to the earth after sunset, 1167

       Plants, why are the seeds of many enclosed in rich juice or pulp, 1172

       Plants, why have some tough curly tendrils, 1173

       LESSON LVI.

       Plants, why have peas tendrils, and beans none, 1174

       Plants, why have grasses, &c., joints or knots in their stalks, 1176

       Plants, what is the circulation of sap in, 1179

       Plants, why does their sap ascend and descend, 1180

       Plants, why do they suffer from the smoke of cities, 1188

       Plants, why is it understood that some of them feel, 1141

       Plants, why is it understood that some of them move, 1142

       Plants, of what elementary substances are they composed, 1143

       Plants, whence do they derive their elementary substances, 1144

       Plants, how do they obtain carbon, 1145

       Plants, how do they obtain oxygen, 1146

       Plants, their decomposition of gases by day and by night, 1147

       Plants, how do they obtain hydrogen, 1147

       Plants, how do they obtain nitrogen, 1148

       Plants, how do they apply their elements to the formation of their structures, 1149

       Plants, how is their nutritive sap applied to their growth and nourishment, 1150

       LESSON LVII.

       Plants, why do they grow, 1154

       Plants, why if we break the stem of a hyacinth do we see a glutinous fluid exude, 1155

       Plants, why if we split the petal of a tulip do we see cells containing different colouring matters, 1156

       Plants, why if we break a pea-shell across do we find a transparent