Horse, why has it no gall-bladder, 1098 Horses, why is the secretion of their eyes thick and glutinous, 1134 Horses and cattle, why do they stretch out their necks, and snuff the air on the approach of rain, 1106 Horse-chestnut, why is it unfit for food, 1203 Hot, when is a body said to be, 77 LESSON XL. Hot water, why does it feel hotter in a metal jug than in an earthenware one, 126 Hot metal, why does it feel hotter than hot wool, 130 Hot metal and hot wool, which would become cold first, 131 Hot and cold bodies, why, when placed near each other, do their temperatures approximate, 331 Humming-tops, why do we see the figures painted on them before they spin, but not while they are spinning, 813 Humming-top, why does it make a noise, 810 Hydrogen, what is it, 49 Hydrogen, will it support animal life, 50 Hydrogen, will it support combustion, 51 Hydrogen, why will it explode, 52 Hydrogen, where does it chiefly exist, 54 Hydrogen, how is it obtained from coal, 57 Hydrogen-gas, is an escape of it dangerous to life, 63 Hydrogen-gas, what proportion mixed with air will explode, 64 Hydrogen-gas, does it rise or fall, 66 LESSON XLI. Hydrogen-gas, what proportion is dangerous if inhaled, 67 Hydrogen-gas, what proportion may be recognised by its smell, 68 Hydrogen-gas, may the use of it be considered dangerous, 70 Hydrogen-gas, what sources of it are there in our dwellings, 69 Hydrogen, why will it burst into flame when coals become heated, 100 Hydrogen, why does hi-carburetted burn with a whiter flame than common coal-gas, 281 Ice, why are blocks of it wrapped in flannel in summer time, 154 Ice, why does it melt, 325 Ice, why does it occupy more space than water, 752 Ignis fatuus, what is it, 318 Incidence, what is the line of, 519 Incidence, what is the angle of, 521 India-rubber, where is it obtained, 1254 Infants, why have they no teeth, 1070 Insects, why have they a large number of eyes, 1083 Insects, why have they long projections from their heads, 1100 Insects, why do they attach their eggs to leaves, 1295 Insects, why do they multiply so numerously, 1300 LESSON XLII. Insects, what is a larva, 1302 Insects, what is a pupa, 1303 Insects, what is a chrysalis, 1304 Insects, what is a nymph, 1305 Insects, why do they abound in decaying substances and in putrid waters, 1313 Insects, why do we see them in tanks of rain-water, 1314 Iron articles, why are they intensely cold in winter, 132 Iron articles, why do they usually feel cool, even when near a fire, 132 Ironing-box, why does the iron sometimes become too large for the box to receive it, 335 Ironing-box, why does the iron enter it, when partially cooled, 336 Iron, why does it rust when wetted, 768 Jew's-harps, why do they give musical sounds, 851