When a solid is melted or a liquid vaporized a large amount of heat becomes latent, insensible to the touch; it disappears as heat. This is one of the most wonderful of the phenomena of nature. It matters not how long the time may be, an hour, a day, a year, or a thousand years after the solid is melted or the liquid turned to vapor, so soon as the vapor returns to the liquid state or the liquid to a solid condition, the latent heat becomes sensible in exactly the same degree in which it previously existed. Let us illustrate with a pound of ice at zero F. Sixteen heat units, or sixteen times as much heat as is required to raise one pound of water one degree, must be absorbed by this pound of ice to raise its temperature to the melting point (32°); and then one hundred forty-four more heat units must be absorbed to so far overcome the tendency of the molecules to adhere, or remain together, that the molecules may roll the one about the other in the liquid form, but with this important difference: the one hundred forty-four units become latent and do not, therefore, cause any increase in temperature, as the sixteen heat units did in raising the temperature of the ice. The large quantity of heat required to change the ice to a liquid is called the latent heat of melting. Any further addition of heat after the melting is complete causes an increase in temperature, and one hundred eighty heat units will raise it to the boiling point. Water boils at 212° at sea level and normal pressure; that is to say, at that temperature the agitation of the molecules of water is so great as to overcome both cohesion and the weight with which the air presses down upon them, and cause them to fly away in the form of steam, which is invisible when confined inside a boiler. But the entire pound of water is not instantly changed to the gaseous condition, for with the sending off of the first few molecules some heat is rendered latent, and more must be supplied or the boiling ceases; in fact the enormous quantity of 964.62 heat units must be supplied to entirely change the pound of water to steam, but at no time does the temperature rise above 212°. As in the former case of changing the solid to a liquid, a large amount of heat becomes latent; in this case it is called the latent heat of vaporization.