Bubbles, why do they fall, after having ascended, 237 Bubbles, why do they display rainbow colours, 499 Bubbles, why are they round, 825 Bubbles, why are they elongated when being blown, 826 Bubbles, why do they close, and become perfect spheres when shaken from the pipe, 827 Bubbles, why do they change their colours in the sunshine, 828 Bubbles, why do they burst, 829 LESSON XI. Burning or supporting combustion, what is the difference, 45 Burning-glasses, why do they appear to set fire to substances, 80 Butterflies, why do they lay their eggs upon cabbage-leaves, 1099 Butterflies' eggs, why do they lie dormant in the winter, 1288 Butterflies, why do they fly by day, 1296 Calms, why do they prevail at the equator, 671 Caloric, what is it, 72 Caloric, what is the source of it, 73 Caloric, what are the effects of it, 74 Caloric, why is it called a repulsive agent, 75 Caloric, how may it be excited to develope heat, 79 Caloric, is there any in ice, snow, water, marble, &c., 89 Caloric, how do we measure the quantity of in any substance, 329 Caloric, how does it travel, 332 Caloric, how do we know that it is caloric which fuses metals, 334 Calves and lambs, why have they no horns, 1069 Camel, why has its stomach a number of distinct bags, 1065 Candles, why do tallow require snuffing, 264 LESSON XIII. Candles, why do composite and wax not require snuffing, 265 Candle, what becomes of it after it is burnt, 269 Capillary blood-vessels, why are they found in every part of the system, 919 Capillary attraction, what is it, 780 Carbonic acid gas, how is it formed, 9 Carbonic acid gas, what becomes of it, 10 Carbonic acid gas, is it heavier or lighter than air, 11 Carbonic acid gas, what are the chief sources of, 16 Carbonic acid gas, what is its effect upon the human system, 21 Carbonic acid gas, what becomes of that formed by combustion, 59 Carbonic acid gas, what proportion is dangerous to life, 60 Carbonic acid, what is it, 798 Carbonic acid, where does it chiefly exist, 799 Carbonic acid, what are its pure states, 800 Carbon, what is it, 18 Carbon, will it produce flame when burnt in oxygen, 106 Carbon and hydrogen, what differences characterise the combustion of, 268 LESSON XIV. Carbon, what is the purest form of it, 277 Card, why do the images on each side blend while a card is revolving, 846 Caterpillars, why do they appear in the spring, 1289 Caterpillars, why do they eat voraciously, 1290 Caterpillars, why do they pass into the state of the chrysalis, 1291 Caterpillars, why do they become torpid in the chrysalis, 1292 Caterpillars, why do they attach themselves to the leaves of plants when in the chrysalis, 1294 Cats, &c., why do they see in the dark, 981 Cats' eyes, why are the pupils of nearly closed by day, 982 Cats, &c., why have they whiskers, 1096