"When a portion of soil," says Johnston, "is dried carefully over boiling water, or in an oven, and is then spread out upon a sheet of paper in the open air, it will gradually drink in watery vapor from the atmosphere, and will thus increase in weight.
"In hot climates and in dry seasons, this property is of great importance, restoring as it does, to the thirsty soil, and bringing within the reach of plants, a portion of the moisture, which, during the day, they had so copiously exhaled."
Different soils possess this power in unequal degrees. During a night of 12 hours, and when the air is moist, according to Schübler, 1000 lbs. of perfectly dry
| Quartz sand will gain | 0 | lbs. |
| Calcareous sand | 2 | " |
| Loamy soil | 21 | " |
| Clay loam | 25 | " |
| Pure agricultural clay | 27 | " |
Sir Humphrey Davy found, that the power of attraction for water, generally proved an index to the agricultural value of soils. It is, however, but one means of judging of their value. Peaty soils and strong clays are very absorbent of water, although not always the best for cultivation.
Sir H. Davy gives the following results of his experiments. When made perfectly dry, 1000 lbs. of a
| Very fertile soil from East Lothian, gained in an hour | 18 | lbs. |
| Very fertile soil from Somersetshire | 16 | " |
| Soil, worth 45s., (rent) from Essex | 13 | " |
| Sandy soil, worth 28s., from Essex | 11 | " |
| Coarse sand, worth 15s. | 8 | " |
| Soil of Bagshot Heath | 3 | " |
"This sort of attraction, however," suggests a writer in the Cyclopedia of Agriculture, "it may be believed, depends upon other causes besides the attraction of adhesion. The power of attraction, which certain substances exhibit for the vapor of water, is more akin to the force which enables certain porous bodies to absorb and retain many times their volume of the different gases; as charcoal, of ammonia, of which it is said to absorb ninety times its own bulk."
Here again, we find in the soil, an inexplicable but beneficent power, by which it supplies itself with moisture when it most needs it.
Warm air is capable of holding more vapor than cooler air, and the very heat of Summer supplies it with moisture by evaporation from land and water. As the air is cooled, at nightfall, it must somewhere deposit the water, which the hand of the Unseen presses out of it by condensation.