288 HOW RAIN IS FORMED. 



for evaporation may take place at all temperatures, eveu from ice. A 

 common little piece of apparatus, often to be seen in the window of the 

 philosophical instrument maker, and known as Wollastou's cryophorus, 

 is a still that works without any fire. It consists of a large glass tube 

 with a bulb at each end, one of which is partly tilled with water; and, 

 all the air having been driven out of the tube by boiling the water, it is 

 hermetically sealed and allowed to cool. It then contains nothing but 

 water and water vapor, the greater part of which re-condenses when it 

 cools. Now, when thus cold, if the empty bulb be surrounded by ice, or, 

 better, a mixture of ice and salt, the water slowly distils over, and is 

 condensed in the colder bulb, and this without any heat being applied 

 to that which originally contained the water. And this shows us that 

 all that is necessary to distillation is that the condenser be kept cooler 

 than the evaporator. 



Nevertheless, at whatever temperature it evaporates, water requires 

 heat, and a large quantity of heat, merely to convert it into vapor ; and 

 this is the case with the cryophorus ; for if the evaporating bulb be 

 wrapped round with flannel, and so protected from sources of heat 

 around, the water cools down until it freezes. That is to say, it gives 

 up its own heat to form vapor. A simple experiment that any one may 

 try with a common thermometer aftbrds another illustration of the same 

 fact. If a thermometer bulb be covered with a piece of muslin, and 

 dipped into water that has been standing long enough to have the saoie 

 temperature as the air, it gives the same reading in the water as in the 

 air. But if when thus wetted it be lifted out and exposed to the air, 

 it begins to sink at once, owing to the evaporation of the water from 

 the wet surface, and it sinks the lower the faster it dries. In India, 

 when a hot wind is blowing, the wet bulb sometimes sinks 40° below the 

 temperature of the air. 



Now this is a very important fact in connection with the formation of 

 rain, because it is owing to the fact that water vapor has absorbed a 

 large quantity of heat, (which is not sensible as heat, but must be 

 taken away from it before it can be condensed and return to the liquid 

 state,) that vapor can be transported as such by the winds for thou- 

 sands of miles, to be condensed as rain at some distant part of the 

 earth's surface. 



1 have said that the quantity of absorbed heat is very large. It 

 varies with the temperature of the water that is evaporating, and is the 

 greater the lower that temperature. From water that is on the point 

 of freezing it is such that 1 grain of water absorbs in evaporating as 

 much heat as would raise nearly 5i grains from the freezing to the boil- 

 ing point. This is called the latent heat of water vapor. As I have 

 said, it is quite insensible. The vapor is no warmer than the water 

 that produced it, and this enormous quantity of heat has been employed 

 simply in pulling the molecules of water asunder and setting them free 

 in the form of vapor, which is merely water in the state of gas. All 



