cles draw as closely together as possible. You can see another in- 

 teresting example of the same thing if you hold the square end of a 

 stick of sealing wax in a flame, for you will find that, as it becomes 

 liquid, the sharp corners disappear and the whole end becomes round 

 like the water-drop. This force which holds particles of liquid to- 

 gether is called cohesion. 



Now, you all can tell me what will happen to awater-drop if it is left 

 for some time where it falls. It will vanish. The first one which we 

 allowed to drop on glass may already have done so. What has be- * 

 come of it? No one saw it going away, and if any one looked at it, he 

 merely noticed that it seemed smaller each time he looked. But has 

 it left nothing whatever behind? If you look closely enough you will 

 probably discover that it has left a thin film or crust. What- 

 ever this crust is, it must have been contained in the water- 

 drop. You should see whether all kinds of water leave the 

 same crust. Try, for instance, drops of spring water, river water, rain 

 water, and also, if you can get them, a small piece of snow and a small 

 lump of ice. We shall not, however, in this lesson, attempt to make 

 a study of the crust that a drop leaves behind. Leaving it for another 

 occasion, let us ask again what became of the water itself. Some of 

 you will be able to tell me that it has become vapor or steam and is 

 scattered in the air around us. It escaped in small particles that were 

 invisible, and in this invisible form they are called water-vapor or 

 steam. To this you may object that the steam formed by a boiling 

 kettle is not iniisible, but look closely and you will find that just 

 where it issues from the spout it is invisible and only becomes visible 

 a little farther on. It is true water-vapor that issues from the spout 

 and what we see a little farther on is not water-vapor, but a little 

 cloud of water-drops. Try putting a spirit flame beneath this cloud, 

 and you will find that the drops are again turned into invisible vapor. 

 Thus we see that one way of turning water into vapor is to heat it, 

 and ihat heat can overcome the force of cohesion, which holds the 

 particles together. It would seem, hofl'ever, that this is not the only 

 way, for if we watch a little cloud from the kettle, we shall see that 

 it gradually turns back again into invisible vapor, which spreads itself 

 throughout the room. Sometimes, however, the cloud rises much 

 higher than at other times, and it seems much less ready to turn into 

 vapor. You will find that this is the case when the kettle has been 

 steaming for a good while in the room and the doors have been closed. 

 Now, this evidently means that vapor forms less readily when there 

 is already a good deal of vapor in the room, and more readily when 

 there is but little vapor in the air. 



