Gold is a valuable metal. It has a high monetary value and is used as a form of payment as well as demonstration of riches, prestige, and power. Gold is a soft metal, thus it is combined with other metals to increase its hardness, such as nickel, copper, palladium, zinc, and so on. The most common additions to gold are silver and platinum. The reason for this is that both elements will harden gold when it is mixed with them, but neither one can harden it beyond the point where it becomes white-white. For example, if you mix gold with silver, you can never go further than 16 layers of silver because once you get past that point, the element's atomic structure changes and it becomes platinous instead of metallic. Some authorities claim that helium should be included in this list of additives since it increases the weight of gold without changing its basic nature. However, helium is a gas at normal temperatures and pressures so it cannot harden anything solid. It is also expensive and extremely hazardous to handle, so it is not added to gold as a rule. Liquids may be added to gold to make alloys with different properties from those of gold itself. For example, you could mix gold with another metal to change the color or hardness of the result. Or you could combine gold with another element, such as nitrogen or carbon, to produce an alloy with new properties.