If an apple is magnified to the size of the earth, then the atoms in the apple are approximately the size of the original apple. The pressure times the area is the force. Let us put twice as many molecules in this tank, so as to double the density, and let them have the same speed, i.e., the same temperature. Then, to a close approximation, the number of collisions will be doubled, and since each will be just as "energetic" as before, the pressure is proportional to the density. If we consider the true nature of the forces between the atoms, we would expect a slight decrease in pressure because of the attraction between the atoms, and a slight increase because of the finite volume they occupy. Nevertheless, to an excellent approximation, if the density is low enough that there are not many atoms, the pressure is proportional to the density. When we compress a gas slowly, the temperature of the gas increases. One exception: helium. Helium, even at absolute zero, does not freeze, unless the pressure is made so great as to make the atoms squash together. If you wish to evaporate water turn on the fan! If we blow on the water so as to maintain a continuous preponderance in the number evaporating, then the water is cooled. Hence, blow on soup to cool it! If we suddenly take the air away from the vessel, then the air molecules will leave more rapidly than they come in, and in doing so will make bubbles. This is very bad for divers. By equilibrium we mean that situation in which the rate at which atoms are leaving just matches the rate at which they are coming back. If we increase the temperature of the salt solution, then the rate at which atoms are taken away is increased, and so is the rate at which atoms are brought back. It turns out to be very difficult, in general, to predict which way it is going to go, whether more or less of the solid will dissolve. Most substances dissolve more, but some substances dissolve less, as the temperature increases. The concept of a molecule of a substance is only approximate and exists only for a certain class of substances. It is clear in the case of water that the three atoms are actually stuck together. It is not so clear in the case of sodium chloride in the solid. There is just an arrangement of sodium and chlorine ions in a cubic pattern. There is no natural way to group them as "molecules of salt." Most substances dissolve more, but some substances dissolve less, as the temperature increases. Observation, reason, and experiment make up what we call the scientific method. Two terms, x-rays and gamma rays, are used almost synonymously. Usually electromagnetic rays coming from nuclei are called gamma rays, while those of high energy from atoms are called x-rays, but at the same frequency they are indistinguishable physically, no matter what their source. The uncertainty of the momentum and the uncertainty of the position are complementary, and the product of the two is bounded by a small constant. An atom has a diameter of about 10^(-8) cm. The nucleus has a diameter of about 10^(-13) cm. Another most interesting change in the ideas and philosophy of science brought about by quantum mechanics is this: it is not possible to predict exactly what will happen in any circumstance. One of the consequences is that things which we used to consider as waves also behave like particles, and particles behave like waves; in fact everything behaves the same way. There is no distinction between a wave and a particle. The fact that a particle has zero mass means, in a way, that it cannot be at rest. The deepest part of theoretical chemistry must end up in quantum mechanics. Nerves are very fine tubes with a complex wall which is very thin; through this wall the cell pumps ions, so that there are positive ions on the outside and negative ions on the inside, like a capacitor. When the impulse reaches the end of the nerve, little packets of a chemical called acetylcholine are shot off (5 or 10 molecules at a time) and they affect the muscle fiber and make it contract - how simple! Enzymes were first called ferments, because they were originally discovered in the fermentation of sugar. An enzyme is made of another substance called protein. Sometimes the same enzyme may control two reactions. It is interesting that the machinery which liberates some hydrogen at one place will take that hydrogen and use it somewhere else. We can take muscle fiber, put it in water, and add GTP (guanosine triphosphate), and the fibers contract, changing GTP to GDP (guanosine diphosphate) if the right enzymes are present. All proteins are not enzymes, but all enzymes are proteins. Proteins are a series, or chain, of different amino acids. There are 20 different amino acids, and they all can combine with each other to form chains in which the backbone is CO-NH, etc. Proteins are nothing but chains of various ones of these 20 amino acids. This is called RNA, but that is not essential.