Because the heat, once latent in the water, but drawn off by the air, has returned to it, and restored the water to its former condition.
"So teach us to number our days, that we may apply our hearts unto wisdom." Psalm xc.
326. Why does water become steam?
Because a larger amount of heat has entered into it than can remain latent in water. The water therefore expands and rises in the form of vapour, or water attenuated by heat.
327. How many degrees of heat are latent, or hidden, in the different states of water?
In thawing ice, 140 deg. of caloric become latent; and in converting the water into steam, 1,000 deg. more of caloric are be taken up. Therefore, ice requires to take up 1,140 deg. of latent caloric before it becomes steam.