Page:Popular Science Monthly Volume 45.djvu/499

Rh saturation of the atmosphere under the conditions above mentioned is more than twice as great at 80° F. as at 50° F.

On the other hand, lowering the temperature of the liquid and vapor by a like amount lessens the number of molecules given off from the liquid surface more rapidly than it lessens the number striking upon the surface. Accordingly, we say that raising the temperature increases evaporation; lowering the temperature increases condensation.

Now, it happens that this same force of cohesion may hold water molecules upon the surface of most solid bodies as strongly as upon the surface of water itself, and in many cases even more strongly. Accordingly, if a solid body of this kind be placed in the atmosphere, the same exchange of water molecules will take place between its surface and the air as between a water surface and the air. In fact, as soon as a layer of water molecules is formed over its surface, it becomes a water surface. Accordingly, if a solid particle be placed in an atmosphere saturated with water vapor and the temperature be lowered, the water molecules will accumulate upon its surface faster than they are driven off, and we say that a precipitation of dew is taking place upon it. The air is accordingly said to reach its dew point when it reaches its point of saturation.

There are other substances which hold fast in a different way the water molecules which strike upon their surface. These substances form either chemical compounds or solutions with water, and in this way remove the water molecules from the places where they strike to the interior of the compound or the solution. Sulphuric acid is a good example of this class of substances. If a vessel of sulphuric acid be placed in a receiver filled with water vapor, the acid holds fast all the water molecules which strike its surface, and sends off no other water molecules to replace them. Since all the water molecules in the receiver will in time come in contact with the acid surface, they will ultimately all be held in a liquid form by the acid. Accordingly, a receiver of moist air can be changed to dry air by allowing it to stand for a sufficient length of time over sulphuric acid.

There are very many other substances which, like sulphuric acid, have the property of condensing the water molecules from a space which is not saturated with them. Such substances are said to be deliquescent, or to gather moisture from the air. Common salt and caustic potash are good examples of deliquescent substances.

There is still another method of producing condensation. If an inclosed space contain water vapor enough to bring it to the point of saturation, and if the volume of the space be decreased without changing the temperature, more molecules will strike