A very familiar instance of this sudden condensation may be seen in the streets of London on any winter day. There may be a couple of omnibus horses, nearly at the end of their day’s work, and quite tired out. Suddenly they are pulled up by the driver, and as suddenly disappear for a moment or two, being concealed in a cloud of moisture proceeding from their bodies. Of course in a hot day there is more of the moisture, but the warmth of the atmosphere prevents it from condensation, and so it is not visible.
One valuable property of the system of evaporation and condensation is its cooling power. Thus it is that a person who is ill with fever tosses about with a burning skin until the pores of the body act, and allow the normal moisture to pass through them. Then the body cools by evaporation, and the patient begins to amend.
So it is that the bather can endure in the Turkish bath a heat so great that a glass of water, if held in the hand, would speedily boil, and a piece of meat be cooked in about the same period. But, if the air were not dry enough to carry off the perspiration, the bather would be scalded to death.
A most valuable adaptation of the principle is shown in the little glass machine for dispersing perfumes in the form of spray. In cases of headache it is almost invaluable, the spray cooling the heated forehead, like magic, and at the same time filling the room with the grateful perfume.
It has even a greater claim to human gratitude, as I can personally testify. I have the strongest objection to a surgeon’s knife, especially when I know, from sad experience, that he is going to make very free use of it. But, on the last occasion, I cared nothing for it, owing to the happy invention called Ether Spray.
The effects were remarkable. First, a delicious cooling of a spot raging with internal fires. Then it was rather colder than I liked. Then it was much colder than I liked. Then it became almost too cold to bear, reminding me of my childhood’s feet on the outside of the Birmingham coach in the depth of winter.
Suddenly all sensation ceased, and the skin became white as parchment. Out came the surgeon’s bistoury, and I looked at him with as calm composure as if he had been whittling a deal plank. There was absolutely no feeling whatever, the local nerves having been temporarily frozen, so great is the power of evaporation. If it ever be my lot again to endure cold steel, I shall have the ether spray.
On the extreme right of the illustration is seen the “Wet-bulb” Thermometer, which carries out the same principle, the thermometer being double, and one bulb being covered with a wet envelope, while the other is dry.
Below is one of the many inventions for making artificial ice, all of them depending on the cooling power of evaporation. Perhaps some of my readers may have seen molten iron poured over the human hand without doing the least harm, or mercury frozen in a red, or rather a white, hot vessel. Both these phenomena are due to the cooling power of evaporation, which is made to act with extreme rapidity, and so absorbs the heat until even mercury is rendered solid, and can be cast in a mould like a leaden bullet.