In 1970s and 1980s, some scholars clearly attributed Brownian motion to the impact of liquid molecules on particles, such as Carponnell, Delsau, Tirion and Negri. Negri (1879), a botanist, thought that fungi and bacteria would not sink even in still air. In connection with the conclusion that gas molecules move at high speed in all directions in physics, he speculated that the flying dust seen on the sun was the result of the impact of gas molecules from all directions. He said: "These tiny dust are thrown around like elastic balls. As a result, they can remain suspended for a long time like molecules themselves." However, negri abandoned this possible way to reach a correct explanation. He calculated the particle velocity when a single gas molecule collided with dust particles elastically, and the result was many orders of magnitude smaller than that actually observed. Therefore, he thinks that due to the irregularity of gas molecular motion, particles can't reach the observed velocity due to interaction, while in liquid, the hypothesis of molecular motion can't be a suitable explanation because of the friction resistance between media and particles and the adhesion between molecules. During the period of 1874- 1880, the work of Pennell, Delsau and Tirion solved the problems encountered by negri. The key point here is that they think that there are density and pressure fluctuations in liquid or gas at the micro scale due to the irregularity of molecular motion and the distribution of molecular velocity. This kind of fluctuation is offset macroscopically. But if the pressure is small enough, this inhomogeneity cannot be offset, and the corresponding disturbance in the liquid can be manifested. Therefore, as long as the particles suspended in the liquid are small enough, they will always oscillate. Carponnell clearly pointed out that the only factor affecting this effect is the size of particles, but he mainly regarded this movement as oscillation, while Delso regarded the movement of particles as irregular displacement according to Clausius' view that molecular movement was attributed to translation and rotation, which was Delso's main contribution. Since then, Guy has made a lot of experimental observations on Brownian motion from 1888 to 1895. Guy's description of molecular behavior is no better than that of Ponnel, and he has no concept of fluctuation. However, what is special about him is that he does not emphasize the physical explanation of Brownian motion, but uses Brownian motion as a tool to explore the essence of molecular motion. He said: "Brownian motion shows that it is not a molecular motion, but some results derived from molecular motion can provide us with direct visible evidence to illustrate the correctness of the hypothesis of thermal nature." According to this view, the study of this phenomenon has played an important role in molecular physics. "Guyi's literature had an important influence, so later Belan attributed the correct explanation of Brownian motion to Guyi. By 1900, F. Exner has finished the final work of the preliminary study of Brownian motion. Thirty years ago, he used many suspensions to do similar research with his father's Exner. He measured the displacement of particles in 65438 0 minutes. Like his predecessors, he confirmed that the particle velocity decreases with the increase of particle size and increases with the increase of temperature. He clearly realized that particles, as macromolecules, joined the thermal motion of liquid molecules, and pointed out that from this point of view, "the relationship between kinetic energy of particles and temperature can be obtained." He said: "This visible movement and its measured values are of further value for us to clearly understand the movement inside the liquid. "The above is the basic situation of Brownian motion research 1900 years ago. Naturally, these studies are closely related to the establishment of molecular motion theory. A great conceptual development of the theory of gas molecular motion established by Maxwell and Boltzmann in the 1960s and 1970s is that the method of tracing a single molecule in detail is abandoned and a large number of molecules are statistically processed instead, which lays a foundation for understanding the origin of Brownian motion. Closely related to the study of Brownian motion is colloid science founded by Graham in the 1960s. Colloid is a dispersion system formed by particles with a particle size between macro-particles and micro-molecules, and Brownian motion is the movement of colloidal particles in liquid medium. 1900 is an important dividing line for the study of Brownian motion. At this point, the appropriate physical model of Brownian motion is obvious, and the remaining problem is to make a quantitative theoretical description. Einstein's Brownian motion theory 1905, Einstein put forward Brownian motion theory according to the principle of molecular motion theory. At about the same time, Smo Ruhoff also made the same achievement. Their theory satisfactorily answers the basic questions of Brownian motion. It should be pointed out that the historical background of Einstein's work was the debate about molecular authenticity in the scientific community at that time. This argument has a long history, which has existed since the theory of atoms and molecules came into being. At the beginning of this century, some people, represented by physicist and philosopher Mach and chemist ostwald, once again criticized the theory of atoms and molecules. They doubt the authenticity of atoms and molecules from the perspective of positivism or phenomenology, making this debate a central issue in the forefront of science. To answer this question, apart from philosophical differences, science itself needs more powerful evidence to prove the real existence of atoms and molecules. For example, the relative atomic mass and molecular mass measured in the past are only relative comparison values of mass. If they are true, then the absolute values of relative atomic mass and relative molecular mass can and must be measured. Such a question needs someone to answer. Because of the above situation, as Einstein pointed out in his paper, his purpose is to "find the most convincing facts that can confirm the existence of atoms of a certain size." He said: "According to the theory of thermomolecular motion, because of thermomolecular motion, an object whose size can be seen by a microscope is suspended in a liquid, and its size can be easily observed by a microscope. Perhaps the movement discussed here is the so-called Brownian molecular movement. He believes that as long as this movement and expected regularity can be actually observed, "it is possible to accurately determine the actual size of atoms." On the other hand, if the prediction about this movement is proved to be incorrect, it provides strong evidence against the popular views of molecular movement. Einstein's achievements can be roughly divided into two aspects. Firstly, according to the principle of molecular thermal motion, it is deduced that it is the statistical average of particle displacement in a certain direction in t time, that is, the root mean square value, and d is the diffusion coefficient of particles. This formula is the inevitable result that seemingly irregular Brownian motion obeys the law of molecular thermal motion. The second aspect of Einstein's achievement is that for spherical particles, it is deduced that η is the viscosity of the medium, A is the radius of the particle, R is the gas constant, and NA is the Avo Gadereau constant. According to this formula, the absolute mass of atoms and molecules can be obtained as long as the accurate diffusion coefficient d or Brownian motion average orientation is actually measured. Einstein estimated the NA value of 3.3× 10 23 by using the diffusion coefficient of sugar in water measured by predecessors. After one year (1906), it was revised to 6.56× 10 23. Einstein's theoretical achievements have found a way to prove the authenticity of molecules, and also satisfactorily expounded the roots and regularity of Brownian motion. The next work is to test the reliability of this theory with sufficient experiments. Einstein said: "I don't want to compare the scarce experimental data I can get with the results of this theory here, but leave it to those who have mastered this problem in the experiment." "I hope a researcher can successfully solve this problem of great significance to thermal theory immediately!" This task put forward by Einstein was successfully completed by Belan (1870- 1942) and Svard Berg respectively. It should also be mentioned that an important experimental progress in the study of Brownian motion at the beginning of this century was that 1902 Siegmund (1865-1929) invented the ultramicroscope, with which the Brownian motion of colloidal particles can be directly seen and measured, which proved the authenticity of colloidal particles. That's why Zigmond won 1925. Swedberg used ultramicroscope to measure Brownian motion. During the period from 1908 to 19 13, Belan carried out experimental research to verify Einstein's theory and determine the Garderot constant of Avo. His work includes several aspects. At the beginning, his idea was that since particles moving in Brownian motion in liquid can be regarded as giant molecules moving in heat, they should follow the law of molecular motion, so as long as a particle's property can be observed experimentally and logically equivalent to the gas law, it can be used to determine Avon Gadereau constant. In 1908, he thinks that suspended particles in liquid are equivalent to "micro-atmosphere of visible molecules", so the height distribution formula of particle concentration (number in unit volume) should have the same form as the atmospheric pressure equation, but the buoyancy of particles should be revised. The formula is ln (n/n0) =-mgh (1-ρ/ρ 0)/kt. Where k is Boltzmann constant, from the relation between k and NA, the formula can also be written as ln (n/n0) =-NAmgh (1-ρ/ρ 0)/rt, and according to this formula, k and na can be calculated from the data of particle concentration height distribution measured by experiments. In order to carry out this experiment, it is necessary to prepare suitable particles first. The preparation method is as follows: firstly, a large amount of water is added into the alcohol solution of the resin to precipitate the resin into small balls of various sizes, and then the resin is fractionated for many times through sedimentation and separation, so as to obtain a fraction with uniform size (such as gamboge balls with a diameter of about 3/4μm). The diameter and density of particles were measured by some fine methods. The next step is to measure the height distribution of particles in the suspension, that is, put the suspension in a transparent and closed dish and observe it with a microscope. After the settlement reaches equilibrium, the particle concentrations at different heights are measured. You can take pictures quickly and then count them. NA can be calculated by measuring height distribution data. Belan and his colleagues changed various experimental conditions: materials (Garcinia Garcinia, Olibanum), particle mass (from 1 to 50), density (from 1.20 to 1.06), medium (water, concentrated sugar water, glycerol) and temperature (from -90). Another experiment of Belan is to measure Brownian motion, which can be said to be a more direct proof of the theory of molecular thermal motion. According to Einstein's formula for spherical particles, as long as the experimental liquid is used, the horizontal projection of particles is observed with a microscope for a selected period of time, and many displacement values are measured, and then statistical average is made. Belan changed various experimental conditions, and the NA value was (5.5-7.2)× 1023. Belan has also used some other methods. The NA values obtained by various methods are: similar air suspension distribution method 6.5× 1023, similar liquid suspension distribution method 6.2× 1023, measurement of disturbance in dense suspension 6.0× 1023 and measurement of translational Brownian motion 6.5×/kloc-0. These results are quite consistent, and they are all close to the modern recognized value of 6.022× 1023. Considering that this method involves many physical assumptions and experimental technical difficulties, it can be said that this method is quite remarkable. Many researchers later measured the NA value according to other principles, which confirmed the correctness of Belan's results. Almost at the same time as Belan, Svedberg (1907) observed the Brownian motion of gold sol with an ultra-micro microscope, and also made outstanding work in determining Avon Gadereau constant and verifying Einstein's theory. It can be said that they were the first people to weigh the atomic mass, so in 1926, Belan and Swedberg won the Nobel Prize in physics and chemistry respectively. In this way, after the discovery of Brownian motion, after more than half a century of research, people gradually approached the correct understanding of it. At the beginning of this century, the theory of Einstein and Smolukhovski, and then the experiment of Belan and Swedberg successfully solved this important scientific problem, and the Avo Gadereau constant was measured for the first time, which provided intuitive and convincing evidence for the real existence of molecules and was of great significance to basic science and philosophy. Since then, the scientific debate about the authenticity of atoms and molecules has ended. As ostwald, the main opponent of primitive atomism, said, "The consistency between Brownian motion and dynamic hypothesis has been successfully confirmed by Belan, which makes even the most critical scientists admit that this is experimental evidence of atomic composition of matter full of space". Mathematician and physicist Peng concluded in 19 13 that "Belan's brilliant determination of the number of atoms completed the victory of atomism". The chemist's atomism is now a real existence. Brownian motion represents a random fluctuation phenomenon, and its theory also has important applications in other fields. For example, the research on the accuracy limit of measuring instruments; Research on background noise in high amplification telecommunication circuits. Brownian motion is different from molecular thermal motion, which is related to temperature and particle number. The higher the temperature, the more intense Brownian motion, the fewer particles and the more intense molecular thermal motion.