For instance, a Fungus epidemic is impossible unless the climatic conditions are such as to favor the dispersal and germination of the spores; and when plants are killed off owi~ig to the supersaturation of the soil with water, it is by no means obvious whether the excess of water and dissolved materials, or the exclusion of oxygen from the root-hairs, or the lowering of the temperature, or the accumulation of foul products of decomposition should be put into the foreground.
But, if no solid be present initially, or if the cooling be rapid, the liquid of composition x becomes supersaturated and may cool till the supersaturation curve is reached at b, and a cloud of A crystals comes down.
This solution is stirred continuously and the temperature falls slowly below the freezing point, till the supersaturation point is reached, or until a crystal of ice is introduced.
If a crystal of the solid be added, the condition of supersaturation is destroyed, and the ordinary equilibrium of saturation is reached by precipitation of solid from solution.
In practical cases of crystallization in nature, it is probable that these phenomena of supersaturation often occur.