Echoes, why do some change the tone and quantity of sounds, 744 Echoes, why are there sometimes several to one sound, 745 Echoes, are they caused only by distant objects, 746 Echoes, what are they, 741 Electricity, what is it, 561 Electricity, why is it so called, 562 Electric fluid, why is it so called, 566 Electrics, what substances are, 567 Electricity, what is positive, 568 Electricity, what is negative, 569 Electricity, what is vitreous, 570 Electricity, what is resinous, 570 Electrical phenomena, what produces them, 571 Electricity, how does its equilibrium become disturbed, 572 Electricity, how does it seek to regain its equilibrium, 573 Electricity, what substances are conductors, 574 Electricity, what substances are non-conductors, 575 Electricity, what substances are insulators, 576 LESSON XXV. Electricity, what is the effect when a current of electricity meets with an insulator, 577 Electric sparks, what is the cause of, 578 Electricity, why does lightning attend it, 584 Electricity, why does it affect the shape of clouds, 613 Electric storms, why are they more frequent in hot than in cold weather, 624 Electric storms, why do they frequently occur after dry weather, 625 Electric storms, why do they purify the air, 629 Electricity, what is Voltaic, 636 Electrical attraction, what is it, 778 Elementary body, what is meant by, 19 Element, what is the most abundant in nature, 756 Elephant, why has it a short unbending neck, 1076 Elephant, why has it a trunk, 1077 Elephants' hind legs, why do they bend forward, 1078 Etna, what are the botanical regions of Mount, 1240 Endogenous stems, what are they, 1280 LESSON XXVI. Endogenous stems, why do they abound in tropical climates, 1281 Endogenous stems, why have they no bark, 1282 Endogenous stems, why do they grow to a great height, 1283 Exogenous stems, what are they, 1279 Exercise, why does it promote health, 1016 Exercise, why does it make us feel warm, 839 Explosions of gas, what is the best method of preventing, 65 Eye-balls, why are they white, 911 Eye-balls, why do they sometimes become blood-shot, 912 Eyes, why are they placed in the sockets of the skull, 929 Eyes, why are we able to move them, 965 Eye, why does the pupil look black, 968 Eye, why is the pupil larger sometimes than at others, 969 Eyes, why have we two, 970 Eyes, why having two, do we see singly, 971 Eyes, why are they provided with eyelids, 972 Eyelids, why are they fringed with eyelashes, 973 Eyes, whence are their humours derived, 976 LESSON XXVII. Eyes, why do we feel inconvenienced by sudden light, 978 Eyes, why if we look upon a bright object and turn away, are we unable to see, 979 Eyes, why are we able to see better after a little while, 980 Eyes, why do cats, bats, owls, &c., see in the dark, 981