Specific gravity usually refers to a material's density when compared to the density of water. This number is given as a ratio which means that there are no units when discussing it. By using a known specific gravity for a particular substance, it is possible to determine what a a sample of the substance is pure or how concentrated it is. In some industries, the density of two substances can be compared with each other instead of with water when determining this measurement.
In most cases, specific gravity is given as the ratio of a certain substance's density compared to the density of the same amount of fresh water at 39 degrees Fahrenheit (4 degrees Celsius). At this temperature, fresh water is at its greatest density, which is 1 gram per milliliter. A substance with a specific gravity lower than that of water will float on top of water, while an object with a higher one will sink. Temperature and atmospheric pressure will alter the specific gravity of a substance, so standard specific gravity values are determined at this temperature and at one atmosphere of pressure, which is the air pressure at sea level.
Aside from standard measures, it is also possible to compare the density two substances with one another instead of comparing them with the density of water. Many different industries use their own sets of density comparisons that help them check concentration. When testing the density of gases, air is often used as a standard, and liquids and solids are often tested against water, though other liquids can be used. It is also possible to measure a value known as the apparent specific gravity of a substance by creating a ratio of the weights of equal volumes of the substance being tested and a reference substance, such as water.
The concept of specific gravity was discovered by the Greek scientist and inventor, Archimedes. When tasked with determining whether the king's new crown was made out of solid gold, Archimedes discovered that he could take the crown and an equal mass of solid gold and see whether the two displaced the same amount of water from a tub. Soon after this discovery, the densities of a number of different substances were calculated so that it would be easy to determine their purity. It is also possible to take an unknown substance and experimentally determine it's density compared to water in order to narrow down the possible materials it could be made from.