“How is it, Uncle George, that there is always a clear space between the
spout of the kettle and the puff of steam?”
It was Frank that asked the question. He had been watching the kettle
boiling for a long time.
“It is the same with the railway engine,” said Tom. “There is always a
big space between the funnel and the puff.”
“That is so,” said Uncle George. “But if you watch carefully, you will
notice that the space between the funnel of the engine and the puff of
steam is not always of the same size.”
[Illustration: Engine puffing on a hot day.]
“No,” said Frank, “I have noticed that the white puff is farther away
from the funnel on a hot day than on a cold day.”
“That is true,” said his uncle. “Perhaps you have also noticed that, as
the engine rushes along, it leaves a long white cloud trailing in the
air behind it. Sooner or later this long white cloud melts away from
sight. It melts away sooner on a hot day than on a cold day. Where does
it go?”
“It goes into the air,” said Frank; “just as the cloud from this kettle
goes into the air of the room.”
“Very good,” said Uncle George. “Tom, will you please fetch me a tumbler
full of cold water, and see that the outside of the glass is quite dry?”
When Tom came back with the glass of cold water, Uncle George wiped it
outside with a clean dry cloth. When he was sure that the outside of it
was dry, he placed the tumbler of water on the table in the middle of
the warm room.
“Now,” he said, “let us try to answer Frank’s question about the clear
space between the spout of the kettle and the puff of steam.
“The fact is,” Uncle George went on, “the white puff which we call
_steam_ is not steam at all. We might just as well call it
‘_water-dust_.’ For it is made up of tiny droplets of water—so tiny that
they float in air. Steam is water in the form of gas. Like the air we
breathe, it cannot be seen. In fact, this water-gas forms part of the
air around us. The clear space between the spout of the kettle and the
puff is made up of hot steam. We cannot see it. As it comes out into the
colder air, it is cooled into the tiny droplets which form the puff. It
is only when cooled into tiny droplets that we can see it.
“If you hold any cold object, such as a knife, in the puff, these water
particles run together and form large drops upon it. The cloud of
water-dust melts away in the room, as Frank told us. What takes place is
this. The tiny droplets, when spread out into the warm air, become real
steam, or ‘water-gas,’ again.
“The outside of this tumbler of cold water was quite dry when I placed
it on the table. Run your finger along it and tell me what you find.”
Frank did so, and said, “Why, it is quite wet now.”
“Yes,” said Uncle George, “it is covered all over with very small drops
of water. Where did this water come from. It could not come _through_
the glass.”
“It is like dew,” said Tom.
“It is dew—_real dew_,” said Uncle George. “The water in the glass is
much colder than the air around it. The film of air next the glass is
cooled, and the ‘water-gas’ which this film of air contains is changed
into water drops.
“The earth is heated during the day by the sun, and the layer of air
next to it becomes filled with water-gas. At night the earth gets cold.
The water-gas, if the night is calm, comes out of the film of air next
to the earth. It settles in the form of tiny drops on everything around.
“When the earth gets very cold, the water freezes as it changes from gas
to water, and instead of dew we have _frost_.”
“Oh, that is why we have frost on the _inside_ of the window panes in
winter,” said Frank.
“That is so,” said his uncle. “The frost on your window pane is the
water-gas of the warm room changed into particles of ice. But let us
come back to our steam puff. We spoke about the long white streak of
water-dust which the engine leaves behind it. Do you know of anything
else like that outside?”
“Oh yes,” said Tom, “the clouds far up in the sky are very like it.”
“They are,” said Uncle George. “In fact, the clouds in the sky and the
cloud behind the engine are just the same kind of thing. They are both
made up of tiny particles of water.
“We have learnt that the streak of cloud left by the steam-engine melts
away quickly on a hot day, also that the puff is farther from the funnel
on a hot day. This shows us that the warmer the air is, the more water
can it take up and hold. We have also learnt that warm air is light and
rises up.[2]
“What happens when warm air, which holds much water-gas, rises up to the
higher and colder parts of the sky?”
“It gets cooled,” answered Frank.
“Yes, and its water-gas gets cooled too. Then we can see it as great
masses of water-dust. These masses we call _clouds_. If these masses of
cloud get further cooled, the tiny water particles run together to form
great drops—as they did on the cold knife. They are now too large and
heavy to float in the air, so they fall to the earth as _rain_.”
[Illustration: A Showery Day.]
Questions and Exercises.
1. If you place a saucer full of water outside on a hot day in the
morning, and go back to it in the evening, you will find the
saucer dry. Where has the water gone?
2. Fill a tin can with a mixture of salt and snow (or chopped ice).
Place it in a warm room. _Frost_ comes on the outside of the
tin. Place a glass jug of water in the same room. _Dew_ is
formed on the outside of the jug. Can you explain this?
3. Let the steam puff of the kettle strike against a cold sheet of
glass or metal, or a slab of stone. What is formed?