248 THE FORMATION OF DEW. 



size the blue appears, showing that little water was held in the 

 veins. What, then, has been for centuries called dew is not dew 

 at all, but the watery juices of the healthy plants. 



But look over dead leaves on a dewy night, and you will see a 

 fine pearly lustre — that is dew. Dead matter gets equally wet 

 when equally exposed, and real dew is not so common as is 

 generally supposed. On many nights on which grass gets wet no 

 true dew is deposited on it, and on all nights, when growth is 

 healthy, the exuded drops always appear before the true dew. 

 The difference between the true and the false dew can easily be 

 detected. The moisture exuded from the leaf veins of the grass 

 — false dew — is always isolated at points situated near the tips of 

 the blades, forming drops of some size ] whereas true dew collects 

 evenly all over the blades. A glance distinguishes the pearly 

 lustre of the dewy film from the glistening diamond drops of the 

 healthy plant's juices. 



But whence comes the dew ? It does not fall from the air. 

 Whence comes it then ? We shall see. Ground a little below the 

 surface is always warmer than the air over it. So long, then, as 

 the surface of the ground is above the dew point, vapour must 

 rise and pass from the land into the air. The moist air so formed 

 will mingle with the air above it, and its moisture will be con- 

 densed, forming dew wherever it comes into contact with a surface 

 cooled below the dew point. In fact, dew rises from the ground. 

 Place some metal trays over the grass, the soil, and the road 

 on dewy nights. You will generally find more moisture on the 

 grass inside the trays than outside ; you will always observe a 

 deposit of dew inside the trays, even when there is none outside 

 at all. This shows that far more vapour rises out of the ground 

 during the night than condenses as dew on the grass and the 

 objects. 



Pieces of iron lying on grass are soon surrounded by richer 

 grass, on account of the moisture which the cold metal attracts 

 from the rising water-vapour. Travellers in Australia and South 

 Africa state that they often found the under-side of their water- 

 proof bedding placed on the ground to be wet after camping out 

 at night. That shows that even in dry countries vapour rises from 

 the ground at night. I remember, when walking in the vicinity of 



