Upon a pleasant day during summer, especially if the sun shines brightly, much aqueous vapour or mist is held suspended in the air, and if the temperature at sunset falls below the dew point, that vapour can no longer be retained in suspension in the air, and falls to the earth. The dew is the vapour of the air. Sometimes it can readily be seen falling in a fine mist resembling rain. It is the humidity of the air deposited upon all surfaces of the earth with which it comes in contact. When the temperature falls below the dew point, or 32°, the dew then becomes converted into frost, and we have a deposit of hoar frost, instead of the dew. It has been remarked that horizontal and flat surfaces exposed to the dew receive a greater deposit than sheltered or oblique surfaces. [Illustration: 9. A dew-laden strawberry leaf with dew jewels set in each serration about the edges] [Illustration: 10. Dew drop caught on vegetable hairs of mullein leaf] [Illustration: 11. The sleeping caterpillar was a good subject and received a copious collection of dew] [Illustration: 12. The surface of a leaf dew-covered] Dew has frequently been quoted as “A shower from heaven,” but this is not literally correct. True, it appears rather mysteriously from a clear sky, and upon a still, cloudless night covers thickly every blade of grass and plant life with seeming raindrops, and that frequently where rain clouds rarely appear, and the rain seldom falls. In such climates, where a rainfall is rare, it is certainly a most beneficial and wise provision, for it gathers upon all herbage and vegetation, in sparkling, refreshing profusion; while it avoids instinctively all barren rocky formations and all things which could not be benefited by its grateful cooling, moisture. Also, in cold, damp climates, where the air is continually saturated with moisture, and where an additional amount is not required, the gathering clouds and the dampness of the chilly atmosphere prevent a radiation of heat from the earth, and the dew never falls in such climates. There are three requisites which appear to be essential for the formation of the dew: First, that the air should be moist; second, that the surface upon which it falls shall be cold, and third, that the sky be clear. Of course the atmosphere always contains a greater amount of moisture after a rainfall, when the air has been greatly cooled. Evaporation is then continually going on among all objects lying near the surface of the earth. Blades of grass and all plants near the ground gradually cool and assume a lower temperature after sunset; they are preparing for the fall of the dew. It has been remarked that certain plants possess greater powers of radiating heat and of expelling moisture through evaporative process than others; upon such plants the dew deposit is always more profuse, while those plants possessing less powers of radiation and evaporation, collect little dew.