There was a famous question asked by Richard Feynman that, suppose one day our whole civilization is supposed to doom. Doomed to disappear and in that case, what is the single sentence which contains the most scientific information in it in the shortest words that we'd like to pass to our next generation, if any, or just to show that we ever existed, intelligence ever existed. What is that sentence? Feynman's answer is that matter is made of atoms. For sure, we know that this sentence is not exactly correct in the sense that if we're talking about conventional atoms, then light is not made of conventional atoms, although light is also quantized. Dark matter is not made of conventional atoms, although it is likely to be quantized. Well, we don't know what it is. Dark energy is not even likely to be made of any kind of particles. However, still, it is extremely important to realize that our daily matter are made of atoms. Now the question is, how do we know? How do we know matter is made of atoms? For sure, with the help of modern technology, we can use our experiments to see the atoms directly. For example, using scanning tunneling [microscopes]. However, this is modern technology and we'd like to know, in the ancient times, hundreds of years ago, in that time, how do we know that matter is made of atoms? Your answer may be that ever since Democritus, we know that matter is made of atoms but that is not a scientific answer. That is a philosophical imagination. Of course, there's also great but there is no scientific evidence, evidence based on observations and experiments at that time that matter is indeed made of atoms. Then what is the first time that we ever know? We ever noticed that things are made of atoms. This theory dates back to 1757 by Franklin. By that time, he already noticed that if you spread a drop of oil onto water surface, that tiny small drop of oil can spread as large as a radius of 100 meters. What that means, how large is a drop of oil? Could be somewhere 10^minus 2 or to 10^minus 3 meters. Let's say 10^minus 2, a pretty big drop of oil. What is the volume in this drop of oil? In rough order of magnitude estimation, let's just cube it. Then his observation is this drop of oil spread on water. The water surface of over 100 meters is covered. Well, that means 100 squared meter and the drop of oil, the volume is roughly the same. That means, what is the width of the layer of that oil film? What is the width? The depth of that oil film is of order this number divided by this number, which is 10^minus 6, and there's addition of minus 4, 10^minus 10 meters. What's the implication? The implication is he discovered a completely new scale. What that means? At a moment the oil film stops spreading corresponds to a new scale. Otherwise, if matter is continuous into infinite small, then the oil film could spread forever. But it stops spreading at a definite scale, which corresponds to the depth of that oil film is 10^minus 10 meters. The fundamental scale means that likely things become not continuous at that scale. In other words, things are made of atoms at the scale of 10^minus 10 meters. This is great observation, but there is a problem to make this great observation into a great experiment. That is, it's very difficult to have an under control situation of 100 meters squared. What if there is wave on water? What if there is wind and rain? Even a fish jump out of the water can slightly change the situation, and what if the oil film is not exactly spherical, there are so many uncertainties which are not under our control. We would like to consider an experiment in our laboratory that under control and 100 meter square is not likely the size of our lab. What can we do? We'd like to make, for example, 0.1 meter square. If we can make the oil film to spread just 0.1 meter square that will be perfect. How can we do that? How can we do that is if we modify this number, 10_minus 2 to 10_minus 4. Then here we modify this number to 0.1 meter suqared. That is perfect, but how can we make 10_minus 4 here, that corresponds to that we take 10 to the minus six drop of oil and spread it onto the surface of water. But now the problem is, have you ever seen anything like 10_minus 6 drop of oil? How can you make 10_minus 6 drop of oil? This idea can come at any moment, but happens to arise in pretty modern era, that what if you first make an oil alcohol solution that you put a drop of oil into alcohol and then when it's made solution, some of the oil can resolve into alcohol and then when we made a solution, we take a drop of that solution and spread it onto water. Then the alcohol simply become solution with water and the oil spread on the surface of water, and that will be of order 0.1 meters. Once you have that, you can measure what is the radius of the atom and it's scale is 10_minus 10 meters. The fundamental scale giving us indication that things are made of atoms. For sure, for such a important statement that things are made of atoms, just one indication is not enough. We'd like to have from every direction, so many directions different indications, and eventually we believe. What are other indications that we can find that world is made of atoms? There are lots of them. For example, Dalton's Law of multiple proportions, which is the beginning of modern chemistry. Dalton notices that we can make two different outcomes, make use of carbon and oxygen. That corresponds CO and CO_2. But at that moment, there is no such notation, we do know that. But at that moment, he has already discovered that if we're given the fixed amount of carbon, then the amount of oxygen used to form these two different types of outcome is 1:2. For one type of outcome, now secretly we know that is CO, and for the other outcome, secretly we know is CO_2, and the ratio of oxygen used for fixed amount of carbon is 1:2. Amazing. Why is amazing? At that time, we have no idea it's CO or CO_2. We just know the ratio, 1:2. How would that be possible? Imagine if we spread butter onto bread. Can we just make two types of butter bread? No. We can make a continuous infinite many possible type of butter bread by spreading different amount of butter on the bread. Similarly, put milk into coffee, you can make infinitely types of latte by different amount of milk into the same coffee. But now, putting oxygen into carbon, you have only two ways to make oxygen carbon. The two ways the amount of oxygen used is 1:2, simple integer number. How does simple integer proportion arises? The world is made of atoms. Once we understand that oxygen and carbon they are not continuous matter, but they're made of atoms. These two outcomes are from the combination of their atoms and this is the moment that we understand they corresponds to something CO and CO_2 and this is the moment that we understand that things are made of atoms. The Franklin's oil film and Dalton's Law of multiple proportions, they're among a lot of evidences that tells us that world is made of atoms. I will put more pieces of evidence into side remarks.