If a drop of ink is added to a glass of clear water, it will gradually spread out and eventually mix completely with the water, indicating that the molecules of one liquid have entered another. Liquid molecules are constantly in motion, or the molecules that make up the liquid are very "active". Generally, the molecules that make up gases are even more "active". For example, once the jasmine you planted blooms, even your neighbors can smell its fragrance; if fish or meat rot, it will cause a foul odor in the surrounding area. In fact, solid molecules or atoms are not very stable either. If you tightly press a very smooth and clean lead plate against a gold plate, after a few months, you will find that the two plates are adhered together. The lead atoms have "moved" into the gold plate, and the gold atoms have also "moved" into the lead plate. If left for 5 years, the gold and lead atoms will actually "infiltrate" into each other by about 1 centimeter. Similarly, the walls and floors where coal has been stored for a long time have become black, which is the result of carbon atoms from coal entering the walls. This phenomenon is called thermal motion, which refers to the irregular motion of a large number of molecules and atoms that make up matter. The higher the temperature of an object, the more intense the thermal motion will be.
A famous phenomenon that proves that liquid and gas molecules move in a random manner is "Brownian motion", discovered by British botanist Robert Brown. In 1827, Brown put pollen into water and then took a drop of this suspension and observed it under a microscope. He curiously found that some small particles separated from the pollen were moving around in the water as if they were possessed, and the direction and speed of each particle's movement changed very quickly, as if they were dancing a kind of irregular dance. Even if the suspension of these small particles is sealed, this movement can be observed at any time of the day or night, in spring, summer, autumn and winter. No matter how long you observe it, this movement will never stop. This same kind of Brownian motion can also be observed in the air, especially in a sunlit room, where dust particles suspended in the air (such as dust) can be clearly observed as if they are also dancing an irregular dance.
How does Brownian motion occur? Originally, the liquid we see as a continuous whole is actually composed of countless molecules, and the liquid molecules are constantly making random movements (thermal motion). When the suspended particles are small enough, the impact of liquid molecules from all directions on them is unbalanced. At one moment, the particles receive a strong impact in one direction, and at the next moment, they receive a strong impact in another direction, causing the particles to constantly change their direction of movement. In this way, the random Brownian motion of the particles is caused.
After the discovery of Brownian motion, people didn't understand why it happened for a long time. In 1877, Delsso correctly pointed out that Brownian motion is caused by the continuous impact of surrounding molecules on the microscopic particles. In 1905, Einstein used statistical methods to satisfactorily explain the regularity of Brownian motion and proved that atoms and molecules do exist. The related experiments and theoretical explanations of Brownian motion have become the foundation of molecular kinetic theory and statistical mechanics.