Lesson 42 How Heat Affects the Absorption of Vapor by the Air

Our early lessons made us familiar with the fact that the air always contains moisture or vapor, said Mr. Wilson, "although the quantity varies from time to time. We are now in a position to inquire into the reason of this.

Any of you may perform this little experiment for himself. Take a towel, weigh it, or get somebody to weigh it for you, dip it in water to wet it thoroughly, then wring it out, and have it weighed again. It will weigh heavier than at first, because of the water it holds. If you hang it up in this condition in the open air it will, after a time, become quite dry, and if you then weigh it, you will find it to be exactly the same weight as at first. The water which it held will have disappeared. If I were then to ask you what had become of the water, you would tell me at once that it had been evaporated, and that the air had sucked up or absorbed the vapor, as a sponge absorbs liquids.

Boiling, as we saw in one of our recent lessons, is essentially evaporation on a rapid and violent scale. The vapor (steam) is formed in masses at a high temperature, and it rises and spreads, by reason of its own tremendous expansive force. The vapor of ordinary evaporation is formed at much lower temperatures, and rises silently and invisibly, forcing its way, molecule by molecule, into the pores between the molecules of the air itself. So much, then, for the presence of vapor in the air: now as to the variation in quantity.

Imagine a glass jar fitted with an air-tight lid and filled with perfectly dry air, at an ordinary average temperature, 60°F. Inside the jar is a small shallow saucer of water, suspended from a delicate spring balance. Now let us see what we have. We have a vessel containing perfectly dry air, and in it there is a small saucer of water. Note what takes place. Evaporation commences immediately, and it is very rapid at first. The air is thirsty, and drinks up moisture eagerly. Our delicate scales would soon tell us that the saucer is getting lighter, because it is being robbed of its water by evaporation.

In a short time, however, the scales would show that less and less water is leaving the saucer. Evaporation is becoming slower and slower, and at last it ceases altogether. The air has finished its drink; it is quite full; it can hold no more. The molecules of vapor have filled up all its pores. We say that the air is saturated—that is, full of moisture. Now, what is the general effect of heating bodies— solids, liquids, or gases? The heat drives their molecules farther apart, by overcoming the force of cohesion. If, therefore, air be heated, and its molecules driven farther apart, it must make the spaces or pores between the molecules larger.

Let us then suppose that we could suddenly increase the temperature of the jar and its air to 90°F. We should see, by the balance again, that there would be a second rapid evaporation from the saucer, which would, after a time, become slower and slower, and at last cease entirely. What is the meaning of all this? The air at the higher temperature is more porous—it has greater capacity for absorbing moisture. Evaporation commences the second time, and goes on until all the pores are filled with moisture. It then ceases entirely, because the air has again become saturated at the higher temperature. Our balance would show us another fact. The amount of water removed by evaporation this time from the saucer would be twice as heavy as at first. That is to say, the increase of 30°F. in the temperature has enabled the air to take up just twice as much moisture as it could hold at the lower temperature.

Suppose we imagine now the opposite process to be at work. As long as the air in the glass vessel remained at 90° Fahr. it would be able to hold all the moisture it had absorbed. We, however, will suppose that it is suddenly cooled again from 90°F. to 60°F. We should see the sides of the vessel become misty, and the interior would be filled with a dense fog. Let us see what all this means. The air is unable, at the lower temperature, to hold all the moisture, and consequently gives up part of it, which is at once condensed into liquid water again.

If we could collect all this water, we should find it to be exactly the quantity which was taken up the second time, when the temperature was raised from 60℉ to 90℉. If we reduced the temperature of the air still lower, it would give off more and more of its moisture, condensed in tiny globules of liquid water. You will now be able to reason out for yourselves why wet clothes on a line dry much more rapidly, as a rule, in warm than in cold weather, and in a dry atmosphere much more quickly than in a damp, humid one."