Anyone who has read "The Legend of Zhen Huan" knows that when the hot summer comes, the emperor takes his concubines to the Old Summer Palace to escape the summer heat. In each concubine's room, a pile of ice cubes will be placed to cool down the environment, which is equivalent

Anyone who has watched " The Legend of Zhen Huan " knows that when the hot summer comes, the emperor takes his harem and concubines to the Old Summer Palace to escape the summer heat. In each concubine's room, a pile of ice cubes will be placed to cool down the environment, which is equivalent to the effect of a modern refrigerator.

You may lament that the people of the Qing Dynasty were really smart. In fact, the ancient Chinese more than 2,000 years ago had learned how to make ice cubes.

There is a saying in "Zhuangzi": "I have learned the way of Master! I can make a cauldron in winter and make ice in summer!" Although the process of "making ice in summer" is not explained in detail here, we can I learned that the "summer ice making" technology already existed in Zhuangzi's time. It was not until the book "Huainan Wan Bi Shu" written by Liu An, the king of Huainan in the Western Han Dynasty, that the process of "making ice in summer" was recorded in detail. The book mentions: "Put the boiling soup into an urn, seal it with new strings, and sink it into the well (well) for three days to turn into ice."

What is worth studying is why "boiling soup", that is, boiling water, is used in "summer ice making" Woolen cloth? Why put it in a deep well? Why three days?

For those of us who have refrigerators and air conditioners in modern times, the ice-making technology is so simple. Of course, we are confused by the practices of the ancients. Even physicists with profound knowledge of physics have to repeatedly study this ice-making method of the ancients in order to supplement the complete history of physics. It is a pity that physicists are stumped by the wisdom of the ancients.

Are we smarter than the ancients? It seems not necessarily.

Chinese physics historian Hong Zhenhuan was one of the first people to study ancient methods of artificial ice. He focused on the freezing point and air pressure. He believes that lowering air pressure will increase the freezing point of water, so water does not necessarily freeze at 0°C. When the temperature in the well is 3-4°C, the freezing point will occur and the water will freeze. However, when he did this experiment in the laboratory, he found that for every reduction in atmospheric pressure, the freezing point of water increased very little, not even 0.01°C. Therefore, Hong Zhenhuan rejected the hypothesis that air pressure causes water to freeze quickly.

is not air pressure, so what is it?

Professor Li Zhichao who studies the history of science and technology at the University of Science and Technology of China speculates that "summer ice making" is achieved through adiabatic expansion and cooling of gases? Li Zhichao and his students conducted an "ice making" experiment. They filled a glass spherical bottle with a small amount of water, sealed it with a rubber stopper, and inserted a glass tube from the rubber stopper into the glass bottle. , at the same time, the glass tube is externally connected to rubber hose . After measuring and comparing the temperature changes of the water in the spherical glass bottle with a thermometer, we came to a conclusion: sealing the boiling water and cooling it, and deflating it through the rubber hose can greatly reduce the temperature of the water .

Why does it sound a bit nonsense to us? A bowl of boiling water left open in the air will cool down in less than 3 minutes in winter, and it will cool down very quickly in a ventilated place in summer. Is a sealed container really that important?

We noticed that "Huainan Wanbi Shu" mentioned that the "boiling soup" is placed inside the "urn" . The "urn" of is no ordinary container. Its main feature is that the mouth of the bottle is very thin. Fill the urn with some water and boil the water. The water vapor will fill the entire container and expel the air originally in the urn. At this time, seal the mouth of the bottle with some finely woven cloth or fabric, and then use a rope to descend to the depths of the well.

In some high mountain areas of the Central Plains, the temperature in deep wells is below 10°C, water vapor condenses when exposed to cold, and the bottle is almost in a vacuum state, and the air pressure inside the bottle is much lower than the air pressure outside the bottle. When the temperature of the urn is about the same as the temperature of well water , taking the container out of the well will quickly cool down the bottle under the principle of decompression and expansion. If done correctly, ice residue will appear in the container.

Although ice slag will appear, it still doesn’t seem to be able to explain how the ancients made ice cubes. But we know that sealing with fabric is a very important condition.

Fortunately, the legacy has been passed down from generation to generation. Although historians of physics enjoy the convenience of cutting-edge technology, they still maintain curiosity and a rigorous spirit of research on how ancient people lived scientifically.

Lei Zhihua, a scholar from Xiangtan University , after many experimental designs and studies, proposed that there were three important influencing factors for ancient people's artificial ice creation.

The first is the relative humidity of the atmosphere. There will be a dehumidification button on our air conditioner remote control. When we dehumidify, the air humidity will become smaller and the room will feel drier. According to the principle of ancient people using urns to make ice, the air humidity becomes smaller and the drier the surrounding air is, the faster the water in the bottle will evaporate.

The second one is the volume of the urn. When the container is larger, the more water vapor it can hold, the higher the probability that the container will be in a vacuum state, and it will be easier to freeze.

The third is the temperature of the well water and the atmosphere. The lower the temperature of well water, the easier it is to freeze. Generally, the temperature of deep wells is between 6℃-10℃. As for why the well water temperature is low? It is related to the freezing point and heat transfer effects we mentioned earlier.

After studying these three important conditions, Lei Zhihua designed a more accurate simulation experiment to create ice.

On August 26, 2005, the temperature near the Chemical Engineering Building of Xiangtan University was about 20°C, and the relative humidity of the air was about 5%. It was a good time to make ice. Lei Zhihua prepared an urn with a capacity of 100L, an electric heating tube, a kerosene hygrometer and fabric, and began to conduct experiments.

He first added a small amount of water to the container, and then heated it with an electric heating tube. The water quickly boiled. After a while, the water vapor filled the entire container, and most of the original air was discharged out of the container. Since the mouth of the bottle is very small, most of the water vapor remains in the bottle. When the water is almost dry, stop heating and immediately seal the bottle mouth with the prepared fabric. Why use fabric instead of plugs? This is for breathability. Professor Lei Zhihua placed the bottle in cold water at 8°C. At this time, air enters the bottle through infiltration. Professor Lei Zhijun observed the thermometer until it stayed on the 0 scale. He opened the jar and took a look. There was a lot of ice residue inside.

Professor Lei Zhijun’s experiment was successful and can basically explain the ice-making process of ancient people.

It can be said that the ancients were quite smart, and they should have gone through many experiments before they could steadily freeze water. Perhaps although they did not understand precise temperature and humidity, they had a better understanding and perception of seasonal solar terms and natural environment than modern people.

A single ice-making technology has stumped so many historians of physics. It can be said that the ancients were smarter than we thought, what do you think?