When the water in a kettle boils, it boils first at the bottom. 当水壶中的水沸腾时, 首先开始沸腾的是底部的水。

　　We cannot see this in a kettle, because the kettle is made of iron. 我们无法看到水壶中的情况, 因为它是铁做的。

　　Let us take a flask or bottle made of thin glass, and fill it half full of water. 我们拿起一个轻薄的玻璃烧瓶或瓶子, 然后装入半瓶水。

　　We will set this flask on an iron stand, and under it we will place a spirit-lamp—that is, a lamp which burns alcohol or spirit of wine. 将烧瓶固定在铁架子上, 然后在下面放一个酒精灯, 也就是燃烧酒精或者烈酒的灯。

　　Soon little bubbles appear on the sides of the warm flask. 很快, 温热的烧瓶壁上就会出现气泡。

　　These are bubbles of the air which was dissolved in the water, and which the heat causes to expand and rise to the top. 这些溶于水的气泡在热量的作用下体积不断增大, 最终浮到上方。

　　As the water gets hotter, we see larger bubbles form on the bottom of the glass flask. 当水变得更热时, 我们便能看到玻璃烧瓶底部出现了许多更大的气泡。

　　These bubbles are full of water-vapour, or steam as we call it. 这些气泡含有大量的水蒸气。

　　Soon these bubbles begin to rise to the top of the water, where they burst. 很快, 这些气泡全部漂浮到了上面, 然后破裂。

　　Then we say that the water boils. 这时, 我们就判断说, 水沸腾了。

　　If we now fit into the neck of the flask a cork through which a glass tube passes, 这时, 如果我们在烧瓶颈部放入一个中间装了玻璃管的木塞,

　　we shall see a jet of steam or water-vapour rushing out of the upper end of the tube. 就会看到一股蒸汽从玻璃管的上方喷涌而出。

　　Let us hold a cold slate in the way of the water-vapour as it escapes. What do we see? 随后, 我们用一块冰冷的石板挡在蒸汽前方。你会看到什么?

　　At once the slate is covered with thousands of tiny drops of liquid water. 很快, 石板上就会出现成千上万颗小水珠。

　　The cold slate has taken heat away from the water-vapour; and as soon as this is done, the vapour returns to its old state of a liquid. 这块冰冷的石板吸走蒸汽的热量；这过程一完成，蒸汽又变成了原来的液体水了。