We have just learned that with electricity we could decompose the compound water into its elements, hydrogen and oxygen. Now we can prove by another experiment that water contains these two elements. If we burn hydrogen gas, or any substance containing hydrogen, water is formed. This can be illustrated by inverting a cool, dry tumbler over a gas flame, which is composed chiefly of hydrogen, and water vapor will collect on the inside. [Sidenote: Properties of water] Though water is widely distributed over the earth, we never find it absolutely pure in nature. All natural waters contain foreign substances in solution. These substances are taken up from the air, or from the earth. Pure water is colorless, tasteless, and odorless. [Sidenote: Why ice floats] On cooling, water contracts until it reaches the temperature of 4° Centigrade (39° Fahrenheit). When cooled from 4° to 0° C. it expands, and the specific gravity, or weight compared with the space occupied by ice, is somewhat less than that of water; hence ice floats. [Sidenote: Rain-water] [Sidenote: Hard water] [Sidenote: Mineral water] The purest water found in nature is rain-water, particularly that which falls after it has rained for some time; that which first falls always contains impurities from the air. As soon as rain-water comes in contact with the earth and begins its course toward the sea, it also begins to take up various substances according to the character of the soil with which it comes in contact. Mountain streams which flow over rocky beds, particularly beds of sandstone, contain very pure water. Streams which flow over limestone dissolve some of the stone, and the water becomes "hard." The many varieties of mineral water from the various springs throughout the country, take their properties from soluble substances with which they come in contact. [Sidenote: Salt water] Common salt is deposited in large quantities in different parts of the earth. Since salt is readily soluble in water, many streams pick up large quantities of it, and as all water courses ultimately find their way to the ocean, the latter becomes a repository for salt with which the earth-water is laden. [Sidenote: Effervescent waters] Effervescent waters all contain some gas, usually carbonic acid gas in solution, and they merely give up or set free a part of it when placed in open vessels. [Sidenote: Sulfur water] Sulfur water contains a compound of hydrogen and sulfur, called hydrogen sulfid or sulfureted hydrogen, which we will refer to in its order later in this lesson. [Sidenote: Distilled water]