By the mid-nineteenth century, the term icebox had entered the American language, but icewas still only beginning to affect the diet of ordinary citizens in the United States. The ice tradegrew with the growth of cities. Ice was used in hotels, taverns, and hospitals, and by someforward-looking city dealers in fresh meat, fresh fish, and butter. After the Civil War (1861-1865),as ice was used to refrigerate freight cars, it also came into household use. Even before 1880, halfthe ice sold in New York, Philadelphia, and Baltimore, and one-third of that sold in Boston andChicago, went to families for their own use. This had become possible because a new householdconvenience, the icebox, a precursor of the modern refrigerator, had been invented.
Making an efficient icebox was not as easy as we might now suppose. In the early nineteenthcentury, the knowledge of the physics of heat, which was essential to a science of refrigeration,was rudimentary. The commonsense notion that the best icebox was one that prevented the icefrom melting was of course mistaken, for it was the melting of the ice that performed the cooling.Nevertheless, early efforts to economize ice included wrapping the ice in blankets, which kept theice from doing its job. Not until near the end of the nineteenth century did inventors achieve thedelicate balance of insulation and circulation needed for an efficient icebox.
But as early as 1803, an ingenious Maryland farmer, Thomas Moore, had been on the righttrack. He owned a farm about twenty miles outside the city of Washington, for which the villageof Georgetown was the market center. When he used an icebox of his own design to transport hisbutter to market, he found that customers would pass up the rapidly melting stuff in the tubs ofhis competitors to pay a premium price for his butter, still fresh and hard in neat, one-poundbricks. One advantage of his icebox, Moore explained, was that farmers would no longer have totravel to market at night in order to keep their produce cool.
The geology of the Earth's surface is dominated by the particular properties of water. Presenton Earth in solid, liquid, and gaseous states, water is exceptionally reactive. It dissolves,transports, and precipitates many chemical compounds and is constantly modifying the face ofthe Earth.
Evaporated from the oceans, water vapor forms clouds, some of which are transported bywind over the continents. Condensation from the clouds provides the essential agent ofcontinental erosion: rain. Precipitated onto the ground, the water trickles down to form brooks,streams, and rivers, constituting what are called the hydrographic network. This immensepolarized network channels the water toward a single receptacle: an ocean. Gravity dominatesthis entire step in the cycle because water tends to minimize its potential energy by running fromhigh altitudes toward the reference point, that is, sea level.
The rate at which a molecule of water passes though the cycle is not random but is a measureof the relative size of the various reservoirs. If we define residence time as the average time for awater molecule to pass through one of the three reservoirs — atmosphere, continent, and ocean— we see that the times are very different. A water molecule stays, on average, eleven days inthe atmosphere, one hundred years on a continent and forty thousand years in the ocean. Thislast figure shows the importance of the ocean as the principal reservoir of the hydrosphere butalso the rapidity of water transport on the continents.
A vast chemical separation process takes places during the flow of water over the continents.Soluble ions such as calcium, sodium, potassium, and some magnesium are dissolved andtransported. Insoluble ions such as aluminum, iron, and silicon stay where they are and form thethin, fertile skin of soil on which vegetation can grow. Sometimes soils are destroyed andtransported mechanically during flooding. The erosion of the continents thus results from twoclosely linked and interdependent processes, chemical erosion and mechanical erosion. Theirrespective interactions and efficiency depend on different factors.