Trains had special container cars with ice to keep meat, milk and other goods cold for long distances on their way to market.

VOA: special.2010.01.11

You're going to have to know when you describe the system, if your system is homogeneous, like your coffee with milk in it, or heterogeneous like water with an ice cube in it So heterogeneous means that you've got different phases in your system.

你要知道,当你描述一个系统时，你的系统是均匀的呢，像掺了牛奶的咖啡，那样还是非均匀的，像放了冰块的水,非均匀系统就是说,在你的系统中有不同的相存在。

麻省理工公开课 - 热力学与动力学课程节选