Tin is the least important of the six useful metals. It is also inferior in many ways to the others in this group of elements, but is tougher than lead and will make a better wire, though not a really good one. It has a whiteness and a luster that are not tarnished by ordinary temperature and is cheap. That is why it is used in making cooking utensils, pans, etc., and for roofs. But the pans, roofs, etc., are not pure tin. They are thin sheets of iron coated with tin. Pure tin would not be strong enough for these purposes, so a sheet of iron is first taken to supply the strength and then covered with tin to improve the appearance of the tin pans and keep them from rusting rapidly. What Is Gravitation? Gravitation is the result of the attraction which every body, no matter what its size, has for every other body. It is a strange force and difficult to explain in plain words. It is what keeps the heavenly bodies in their paths. Every one of the planets is held in its path by gravitation and every object on each of the planets is kept on the planet by gravitation. We can come nearer understanding gravitation by studying the effect of the attraction of gravitation on our own earth and the objects on it. When you throw a ball or a stone into the air it is the attraction of gravitation that causes it to come back. If this were not so the stone would go on up and up and would keep on going forever. If it were not for this wonderful force you could jump into the air and just keep on going up with nothing to bring you back. The reason you do not pull the earth toward you is because the body or mass with the greater bulk has always the greater pulling power. This is a wonderful force. It cannot be produced nor can it be destroyed or lessened. It just is. It acts between all pairs of bodies. If other bodies come between any pair of bodies the attraction of gravity between the two outside bodies is neither lessened or increased, and yet each of the outside bodies will have an independent attraction or pull on the body which is in between. No particle of time is spent by the transmission of the force of gravity from one body to another, no matter how far apart they may be. The only effect that distance has on the attraction of gravitation is to lessen its force. Any body which is being pulled through gravity toward another body would fall toward the center of the attracting body if all the force of attraction from all other bodies were removed. What Is Specific Gravity? Specific gravity is the ratio of weight of a given bulk of any substance to that of a standard substance. The substances taken as the standard for solids and liquids is water, and air or hydrogen for gases. Since the weights of different bodies are in proportion to their masses, it follows that the specific gravity of any body is the same as its density, and we now generally use the term “density” instead of specific gravity.