Sodium was discovered in 1807 by Sir Humphrey Davy in London. Sodium is a soft, silvery coloured metal which, like other members of the alkali group of metals, is extremely reactive. As for the other members of the group, it can be prepared by electrolysis of the fused halide or hydroxide. Along with potassium, it is one of the more common alkali metals, there being 28300 ppm in the earth's crust, primarily as the carbonate, chloride or nitrate. Molten sodium is used as a heat exchanger in certain types of nuclear reactors and as a reagent in the chemicals industry. Some of the sodium salts e.g. NaCl and NaCO₃) are more important than the metal itself due to the variety of applications for which they can be used. Sodium is an essential element for all living species, including humans, although there is an element of controversy concerning the amount required. An average human body contains around 100 gms of sodium which are lost in various ways and have to be replenished. The average human consumes approximately 10 gms of salt per day although only around 3 gms are actually needed and an excess can contribute to high blood pressure. Sodium performs several functions within the body including the regulation of the water content in the blood and tissue and the transmission of electrical impulses.