第70课 蒸发作用

　　Let us fill a small kettle with water, and place it on the fire. The water will soon begin to boil. After an hour or so we shall find the kettle quite empty, although no one has touched it since it was placed on the fire.

　　我们找上一个水壶, 灌满水, 放在火上。很快, 水就会沸腾起来。经过一个小时左右, 我们会发现水壶已经空空如也。但是水壶放到火上之后, 根本没有人动过。

　　What has become of the water which we poured into the kettle? It has all been changed into a vapour or gas, called steam, which has passed out of the mouth of the kettle and mixed with the air in the room. Because the liquid water has changed into vapour, we say that it has evaporated.

　　那么, 我们倒进去的水去了哪里呢? 它们变成了水蒸气, 经过壶嘴跑了出去, 并且与房间里的空气混合在了一起。我们将这种液态水变成水蒸气的过程称为蒸发作用。

　　But we need not put water on the fire to make it evaporate. Let us place a saucer full of water on the mantel-piece, or out in the garden if the weather is fine. In a few hours the water will have changed into water-vapour and mingled with the air, and the saucer will be quite dry. We cannot see the water-vapour in the air, for it has no colour, and is quite transparent. The water in the saucer will evaporate much more slowly than that in the kettle, and we cannot see rising from it any of the white wreaths which we usually call steam.

　　不过呢, 我们并不需要将水放在火上才能实现蒸发作用。现在, 我们将一碟水放在壁炉架上, 如果天气好, 也可以放在花园中。几个小时之后, 这一碟水就将变成水蒸气与空气混合在一起, 而碟子也就变得完全干燥。我们无法看到空气中的水蒸气, 因为它没有颜色而且完全透明。与水壶相比, 碟子中的水蒸发速度要慢上很多, 而且我们也无法观察到通常被称为水蒸气的那种白色气圈。

　　The hotter the place is in which we put the water, the more quickly will it evaporate. On a hot, dry, and windy day, water placed in the open air evaporates very quickly indeed. Your mother will tell you that such days are the best for drying clothes that have been washed. The water soon evaporates, and leaves the clothes quite dry.

　　水放的地方越热, 蒸发速度就越快。如果遇到炎热、干燥且有风的天气, 放置在户外的水很快就会蒸发。妈妈一定会告诉你, 这样的天气最适合晾晒刚刚清洗过的衣服。水分将会很快蒸发, 而衣服就变得非常干爽。

　　Some liquids evaporate more quickly than others. Alcohol, or spirit of wine, evaporates more quickly than water. Dip one finger into alcohol, and another finger into water, and then wave both in the air.

　　不同液体的蒸发速度存在一定差异：酒精比水的蒸发速度要快上不少。你可以用一根手指沾一些酒精, 另一根手指沾上水, 然后分别在空气中挥动。