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 membrane, 1157 Plants, why if we cut through a cabbage-stump do we find a tough coating enveloping a cellular substance, 1158 Plants, why are their seeds formed within the corollas of flowers, 1168 Plants, why does the flower of the poppy turn down during the early formation of seed, 1169 Plants, why have those of the pea-tribe a folding blossom, 1170 Plants, why are leaf-buds enclosed in scales which fall off as the leaves open, 1171 Plants, what is meant by "species" of, 1209 Plants, what is meant by "genus" of, 1210