ers through much of the year, refrigeration changed the whole nature of the American diet.

The most widespread early refrigeration technology depended on compressed ammonia gas, which easily produced desired drops in temperature for effective food storage. But ammonia (like other refrigerant gases such as sulfur dioxide and methyl chloride) had serious problems. For maximum efficiency, it had to attain high pressures before being released, which increased the likelihood that the compression equipment might fail. Accidental explosions were frequent, and the toxic nature of the gas caused a number of fatalities. Toxicity and the need for large expensive compressors kept mechanical refrigeration from making headway with retail customers, who represented an immense potential demand. That is why Thomas Midgely Jr.'s 1931 invention of Freon 12 represented a revolution for the refrigeration industry. Midgely, working at the request of the General Motors Frigidaire division, developed the new chlorinated fluorocarbon as the perfect alternative to all other refrigerant gases then on the market.

Nonflammable, nonexplosive, noncorrosive, and nontoxic, the various forms of Freon gas seemed the perfect technical solution to a host of environmental and safety problems. They also required less pressure to produce the desired cooling effect, so compressors could be smaller and less expensive. Freon soon came to dominate the market for refrigeration and opened up new retail markets because of its diminished capital requirements. Previously, consumers had bought their refrigerated food at the store just before eating it, since efficient and reliable household refrigeration was not generally available. Now American households could own their own refrigerators, making it possible for the food industry to shift much of its marketing apparatus toward selling chilled food in retail-sized packages. Frozen foods burst onto the American marketplace in the 1950s, as did fresh vegetables, dairy products, and other foods that are today accepted as ordinary parts of the national diet. Although European countries were slower to adopt these technologies, they too eventually followed suit.

No less importantly, the nontoxicity of Freon made it possible for refrigeration technology to be applied to the ambient cooling of buildings, so that air conditioning came to be an ever more important market for the gas. Air conditioning had been used in specialized industrial applications ever since Willis H. Carrier's use of the technique for a climate-controlled lithography plant in 1902. The introduction of Freon meant that air conditioning suddenly became much cheaper and safer in a way that allowed it to

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