1.When the water in a kettle boils，it boils first at the bottom.We cannot see this in a kettle，because the kettle is made of iron.Let us take a flask or bottle made of thin glass，and fill it half full of water.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.
2.Soon little bubbles appear on the sidesof the warm flask.These are bubbles of the air which was dissolved in the water，and which the heat causes to expand and rise to the top.As the water gets hotter，we see larger bubbles form on the bottom of the glass flask.These bubbles are full of water-vapour，or steam as we call it.Soon these bubbles begin to rise to the top of the water，where they burst.Then we say that the water boils.
3.If we now fit into the neck of the flask a cork through whicha glass tube①passes，we shall see a jet②of steam or water-vapour rushing out of the upper end of the tube.
①Tube，a narrow pipe；any round thing hollow inside.
②Jet，stream；spout.
4.Let us hold a cold slate in the way of the water-vapour as it escapes.What do we see？At once the slate is covered with thousands of tiny drops of liquid water.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.
5.Thus by heating liquids we cause them to evaporate，or change into vapours or gases；and by taking away heat from these gases，or cooling them，we turn them into liquids once more.
6.We saw，in the lesson on steam in your last book，that whenwater changes into water-vapour，it requires a great deal more room.And so，when the water-vapour turns back into liquid water again，it takes up much less room.It needs seventeen hundred pints of water-vapour or steam to produce one pint of liquid water.
7.It is certain，then，that the particles of the liquid must be very much closer together than the particles of the vapour；and a pint of the liquid will therefore weigh a great deal more than a pint of the vapour.
STILL（蒸馏器）
8.Now when the particles of any substance are packed closely together，we say that the substance is dense.So when we cool the steam，and bring its particles close together in the form of water，we are said to condense it，or make it dense；and the process①ofdoing this we call condensation.
9.Let us connect the tube which leads from our flask of boiling water with a twisted or spiral piece of glass tubing which is surrounded by cold water.As the steam passes along the inside of this cold spiral tube it is condensed；its particles are brought closer together，and it therefore changes into liquid water，which falls in drops from the outer end of the spiral tube.
10.When we change a liquid into vapour，and cool this vapour till it is liquid again，we are said to distil the liquid，and theapparatus②for doing this is called a still.You see in the picturea simple form of still.By means of it we can turn salt water into fresh.The salt itself does not turn into vapour，and when we cool the vapour of salt water we get perfectly③fresh water.
