[Pointing hand symbol] _The numbers refer to the Questions. The Index Lessons do not correspond with the Chapters, but are designed to bring together in their alphabetical connection, all the Questions and Answers upon each particular subject included in the work._ LESSON I. Acids, vegetable, whence are they obtained, 1256 Actinism, what is it, 552 Actinism, what effect has it upon vegetation, 559 Actinism, at what season of the year is it most abundant, 560 ærial spectra, what is the cause of, 527 Affinity, what is the attraction of, 777 Air, why do we breathe it, 4 Air, what is the composition of it, 12 Air, what is its state after it has been breathed, 13 Air, is that sent from the lungs light or heavy, 14 Air, is it a good or bad conductor of heat, 150 Air, why is a still summer said to be sultry, 171 Air, why does it feel cooler when in motion, than when still, 175 LESSON II. Air, when is it hot enough to destroy life, 179 Air, why is it often excessively hot in chalk districts, 219 Air, is it heated directly by the sun's rays, 221 Air, why does it ascend the chimney, 240 Air, why does it fly through the doors and windows towards the fire-place, 241 Air, what does the motion of it in our rooms illustrate, 242 Air, why does it become charged with watery vapour, 346 Air, when is it said to be saturated with vapour, 430 Air, what proportion of water is air capable of holding in the form of vapour, 431 Air, what is its weight relative to that of water, 647 LESSON III. Air, which is heavier, dry or vaporised, 697 Air-pistol (or pop-gun), why does pressure applied to the handle propel the cork, 854 Air-pistol, why must the handle be drawn out before the cork is placed in, 855 Air, why does fresh impart a healthy appearance, 915 Albumen, what is it, 899 Alkalies, what are they, 36 Amber, or electrum, what is it, 564 Animals, how is their greater warmth in winter provided for, 168 Animal forms, why are there so many, 1029 Animal furs, why do they become thicker in the winter, 1035 Animals, why have carnivorous, long pointed teeth, 1038 LESSON IV. Animals with long necks, why have they large throats, 1049 Animals, why cannot flesh-eaters live upon vegetables, 1071 Animals, why can ruminating recover the food from their paunches, 1089 Animals, why can ruminating keep the chewed from the unchewed food in their stomachs, 1090 Animals, why do the smaller animals breed more abundantly than the larger ones, 1094 Animals, domestic, why may change of weather be expected when they are restless, 1107 Animals, plants, and minerals, what are the differences between them, 1140 Animals that graze, why do they crop the tender blades, but leave the tall grass, 1287 LESSON V. Animals, distribution of, 1326 Arms and hands, why do we see blue marks upon them, 896