书城外语英国学生科学读本
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第94章 冷凝(2)

11.Our own breath contains water-vapour.We have only to breathe upon some cold object,such as a looking-glass,to condense the vapour in our breath.The mist which we see upon the cold glass consists of thousands of tiny particles of water.On a frosty morning the cold air condenses the water-vapour in our breath as soon as it leaves the mouth,and makes it look like a puff of steam;so that each child looks like a little steam-engine,as he runs along the street on his way to school.

12.The little white clouds which come in puffs from the engines on the railway,or which rise from the kettle,or from our breath,are not made of true steam or water-vapour at all.They really consist of a great many little specks of condensed water.

①Process,action.

②Apparatus,instruments for doing some work;machinery.

③Perfectly,quite;entirely.

USES OF STEAM

(蒸汽的用处)

We call them steam,but true steam or water-vapour cannotbe seen.If you look quite close to the top of an engine‘s funnel,or to the spout of a kettle which is boiling on thefire,you will see a clear space between it and the white cloud.This clear space is filled with the true steam or water-vapour.Only after the cold air has condensed and changed the true vapour into little particles of water,or water-dust,can itbe seen.This water-dust is what we commonly①call steam.

①Commonly,usually.

SUMMARY

When water is heated,air-bubbles first rise through it,and then bubbles of water-vapour or steam.When these steam bubbles burst the water boils.A cold substance held in the steam changes it into small drops of liquid water by taking away its heat.A liquid is denser than a gas.The process of turning a gas into a liquid is therefore called condensation.When a liquid is first changed into a gas,and then back again to a liquid,it is said to be distilled.The apparatus for doing this is called a still.By this means we can get fresh water from salt water.True vapour is invisible.The clear space close to the mouth of a steam pipe is filled with invisible vapour.The cold air changes this into a white cloud.The white clouds we call steam are made up of small particles of condensed water-vapour or water-dust.

【中文阅读】

1.当水壶中的水沸腾时,首先开始沸腾的是底部的水。我们无法看到水壶中的情况,因为它是铁做的。我们拿起一个轻薄的玻璃烧瓶或瓶子,然后装入半瓶水。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。

2.很快,温热的烧瓶壁上就会出现气泡。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。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。这些气泡含有大量的水蒸气。很快,这些气泡全部漂浮到了上面,然后破裂。这时,我们就判断说,水沸腾了。

3.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,就会看到一股蒸汽从玻璃管的上方喷涌而出。

4.随后,我们用一块冰冷的石板挡在蒸汽前方。你会看到什么?很快,石板上就会出现成千上万颗小水珠。

5.因此,可以看出:我们能够通过加热液体使得它们蒸发,变成水蒸气或气体;也能够通过消除气体中的热量或者冷却,将它们再次变为液体。

6.大家通过上一本书的学习了解到:当水变成水蒸气之后就需要更多空间。那么当水蒸气再次变成液体的时候,它所占据的空间就会小很多。1700品脱的水蒸气只能转化为1品脱的液态水。

7.毫无疑问,水的分子比水蒸气的分子密度要大上很多;所以一品脱的水远比一品脱的水蒸气要重很多。

8.不论是什么物质,如果它的分子紧密地拥挤在一起,我们就说这种物质密度很大。因此,当我们冷却水蒸气,将水蒸气的分子紧压到一起,转化为液态水时,我们就是对它进行凝结,或者说增加它的密度。这个过程就被称作冷凝。

9.让我们将来自沸水烧瓶的玻璃管,连接到冷水之中的弯曲或螺旋状玻璃管之上。当水蒸气沿着清凉的螺旋状玻璃管前进时,它就被冷凝了;随着分子密度的增加,水蒸气变成了水,从螺旋状玻璃管的外端流下。

10.将水加热成水蒸气,然后对其进行冷却直至再次液化,这个过程就是对液态水进行蒸馏,而它所需要的设备就叫做蒸馏器。通过这种方法,就能够将含有盐分的水转化为淡水。而这其中的盐分不会转化为水蒸气,我们对这种水进行冷却,就可以得到纯净的淡水①。

11.我们的呼吸中就带有水蒸气。只需朝着梳妆镜这样清凉的物体呼上一口气便能够冷凝我们呼吸中的水蒸气。我们在清凉的玻璃上看到的那层水雾包含有成千上万颗小水珠。在严寒的清晨,当水蒸气离开我们嘴巴的瞬间就会被冰冷的空气冷凝,形成一股蒸汽团;如此一来,当孩子们在街道上奔跑着前往学校的时候,每个人都像是一台蒸汽机。

12.火车蒸汽机与我们的呼吸所喷出的白色蒸汽团并非是真正的水蒸气。这其中实际上包含有很多冷凝水微粒。我们称之为水蒸气,然而,肉眼根本无法看到真正的水蒸气。仔细观察蒸汽机烟囱的顶部或者火焰上正在沸腾的水壶喷口,就会发现它与白雾之间有一片明显的空隙。这片空隙就充满了真正的水蒸气。只有当清冷的空气对真正的水蒸气进行冷凝,将其变为细小的水分子或者水雾以后,我们才能看到这种物质。这种水雾就是人们常说的蒸汽②。

译注

①现代社会的冷凝器在空调和锅炉上应用得最多。

②前面的注释已经论述了“水蒸气”和“蒸汽”的关系。这种白雾,前面也提到过,属于混杂着液态水的“湿蒸汽”,而水壶喷口附近的真正水蒸气,属于“干蒸汽”,也叫“过热蒸汽”。在某些工业领域,过热蒸汽是非常危险的,因为无法直接看到,容易造成烫伤。

CLOUDS AND RAIN