When water is changed into steam, what becomes of the steam? The steam passes off into the air. What does the air do with the steam? The air condenses or changes it into very small particles of water, so that it is seen like a fog or a cloud. Can steam be seen? No; steam is like the air in this respect, and cannot be seen. How can we prove this? When water is boiled in a glass vessel, no steam is visible, although it is there. Cannot we see the steam coming from the spout of a tea-kettle? No; we do not see anything for half an inch or more from the spout. Why does it become visible farther away? Because the air condenses it, and then we see it in its cloud-like state. How much steam will one gallon of water make? One gallon of water will make seventeen hundred gallons of steam. When this steam is confined in a vessel, what does it try to do? It tries to expand or spread out in every direction. Does it expand with much force? Yes; it expands with so much force as frequently to burst the vessel containing it. Why is the lid of the tea-kettle lifted up when the water boils? Because the steam expands and raises the lid, so that it can escape. If the lid were fastened down, what would result? The steam, in trying to escape, would burst the tea-kettle. Why does water run out of the spout of the tea-kettle? Because the steam in the tea-kettle presses on the water with force enough to cause it to run out. Why does steam burst vessels in which it is confined? Because the vessels are not strong enough to keep the steam from expanding. How is the boiler of a steam-engine kept from bursting? By means of a safety-valve. What is a safety-valve? It is a valve or opening through which steam may escape. What prevents all of the steam from escaping through the valve? The valve is kept closed by a weight, which prevents the escape of the steam until a certain quantity of it is formed in the boiler; it then lifts up the valve, and a portion of it escapes, the same as it does with the lid of the tea-kettle. Evaporation. Does a wet blanket, when hung in the air, remain wet? No; it soon becomes dry. What becomes of the water in the blanket? The water passes off into the air. What is this process called? It is called _Evaporation_. What is meant by evaporation? It is the gradual change of a liquid into a vapor. When water is left in a shallow vessel, what becomes of it? The water evaporates or passes off into the air. Do liquids generally evaporate? Yes; when exposed to the air, nearly all liquids evaporate. Does water change into steam when it evaporates? Water does not change into steam when it evaporates. How, then, does water evaporate? It passes off into the air in particles or drops so very small as not to be visible. Is evaporation always going on around us? Yes; wherever there is water exposed to the air, evaporation is taking place.