32 MR JOHN AITKEN ON DEW. 



evaporation from the whole surface of the leaf, the drop grows ; but if the 

 supply is less, it does not form, or if formed, it decreases in size. The rate of 

 the supply will evidently depend on the kind of plant and the amount of its 

 vital activity at the time. The formation of drops on plants that exude 

 moisture will therefore depend on the rate of supply, the humidity of the air, 

 and the velocity of the wind. It is not easy to get a satisfactory experimental 

 answer to this question, on account of the soil near the grass tending to 

 moisten the air over it. A small turf placed in an elevated position in the 

 centre of a room has been observed to have drops on it, when there was a 

 difference of more than one degree between the wet and the dry bulb thermo- 

 meter hung alongside. As the drops are exuded at the tips of the blades, 

 it is probable the air in contact with them was not much moistened by the 

 small area of soil underneath. 



These observations entirely do away with the explanation usually given of 

 the tendency of grass to get wet early and heavily on dewy nights. It has 

 generally been explained by saying that grass is a better radiator than most 

 substances, and therefore cools more, and sooner, than other bodies. We 

 now see that those drops that first make their appearance on grass are not 

 drops of dew at all, and their appearance depends, not on the laws of dew, but 

 on those of vegetation. Hence the varied distribution of moisture on plants 

 and shrubs on dewy nights. 



We have seen that much of the moisture that collects on plants at night 

 does not form like dew on dead matter. Dead matter gets equally wet where 

 equally exposed, and the moisture does not collect on it in isolated drops, as 

 it does on plants. Those drops which appear on grass on clear nights are not 

 dew, and they make their appearance on surfaces that are not cooled to the 

 dew-point. If the radiation effect continues after these drops have been 

 forming for some time, true dew makes its appearance, and now the 

 plants get wet all over their exposed surfaces in the same manner as dead 

 matter. This latter form of wetting or true dew is of rarer occurrence than 

 we might at first imagine. On many nights on which grass gets wet, no true 

 dew is deposited on it; and on all nights, when vegetation is active, the exuded 

 drops always make their appearance before the true dew ; so that when we 

 walk in early evening over the wet lawn, it is not dew that we brush off the 

 grass with our feet, but the sap exuded by the plant itself. The difference 

 between these exuded drops and true clew can be detected at a glance. The 

 moisture exuded by grass is always excreted at a point situated near the tip 

 of the blade, and forms a drop of some size, which may form while the rest 

 of the blade is dry, but true dew collects evenly all over the blade. The 

 exuded liquid forms a large glistening diamond-like drop, whereas dew coats 

 the blade with a fine pearly lustre. 



