38. The electric arc 233 39. Short circuits and fuses 240 40. Electromagnets 247 9. MINGLING OF MOLECULES 259 41. Solutions and emulsions 259 42. Crystals 265 43. Diffusion 268 44. Clouds, rain, and dew: Humidity 274 45. Softening due to oil or water 290 10. CHEMICAL CHANGE AND ENERGY 293 46. What things are made of: Elements 293 47. Burning: Oxidation 312 48. Chemical change caused by heat 323 49. Chemical change caused by light 326 50. Chemical change caused by electricity 335 51. Chemical change releases energy 340 52. Explosions 342 11. SOLUTION AND CHEMICAL ACTION 349 53. Chemical change helped by solution 349 54. Acids 351 55. Bases 355 56. Neutralization 360 57. Effervescence 365 12. ANALYSIS 370 58. Analysis 370 APPENDIXES: A. The Electrical Apparatus 379 B. Construction of the Cigar-box Telegraph 381 INDEX 383 COMMON SCIENCE CHAPTER ONE GRAVITATION SECTION 1. _A real place where things weigh nothing and where there is no up or down._ Why is it that the oceans do not flow off the earth? What is gravity? What is "down," and what is "up"? There is a place where nothing has weight; where there is no "up" or "down"; where nothing ever falls; and where, if people were there, they would float about with their heads pointing in all directions. This is not a fairy tale; every word of it is scientifically true. If we had some way of flying straight toward the sun about 160,000 miles, we should really reach this strange place. Let us pretend that we can do it. Suppose we have built a machine that can fly far out from the earth through space (of course no one has really ever invented such a machine). And since the place is far beyond the air that surrounds the earth, let us imagine that we have fitted out the air-tight cabin of our machine with plenty of air to breathe, and with food and everything we need for living. We shall picture it something like the cabin of an ocean steamer. And let us pretend that we have just arrived at the place where things weigh nothing: