These observations and experiments establish the fact that the drops which first make their appearance on grass on dewy nights are not dew-drops at all, but the exuded watery juices of the plants.
If now we look at dead leaves we shall find a difference of formation of the moisture on a dewy night: the moisture is spread equally over, where equally exposed. The moisture exuded by the healthy grass is always found at a point situated near the tip of the blade, forming a drop of some size; but the true dew collects later on evenly all over the blade. The false dew forms a large glistening diamond-drop, whereas the true dew coats the blade with a fine pearly lustre. Brilliant globules are produced by the vital action of the plant, especially beautiful when the deep-red setting sun makes them glisten, all a-tremble, with gold light; while an infinite number of minute but shining opal-like particles of moisture bedecks the blade-surfaces, in the form of the gentle dew—
“Like that which kept the heart of Eden green
Before the useful trouble of the rain.”
CHAPTER IV
HOAR-FROST
All in this country are familiar with the beauty of hoar-frost. The children are delighted with the funny figures on the glass of the bedroom window on a cold winter morning. Frost is a wonderful artist; during the night he has been dipping his brush into something like diluted schist, and laying it gracefully on the smooth panes.
And, as you walk over the meadows, you observe the thin white films of ice on the green pasture; and the clear, slender blades seem like crystal spears, or the “lashes of light that trim the stars.”
You all know what hoar-frost is, though most in the country give it the expressive name of “rime.” But you are not all aware of how it is formed. Hoar-frost is just frozen dew. In a learned paper, written in 1784, Professor Wilson of Glasgow made this significant remark: “This is a subject which, besides its entire novelty, seems, upon other accounts, to have a claim to some attention.” He observed, in that exceptionally cold winter, that, when sheets of paper and plates of metal were laid out, all began to attract hoar-frost as soon as they had time to cool down to the temperature of the air. He was struck with the fact that, while the thermometer indicated 36 degrees of frost a few feet above the ground and 44 degrees of frost at the surface of the snow, there were only 8 degrees of frost at a point 3 inches below the surface of the snow. If he had only thought of placing the thermometer on the grass, under the snow, he would have found it to register the freezing-point only. And had he inserted the instrument below the ground, he would have found it registering a still higher temperature. That fact would have suggested to him the formation of hoar-frost; that the water-vapour from the warm soil was trapped by a cold stratum of air and frozen when in the form of dew.