ALKALOIDS, the name of a group of organic bodies that possess alkaline properties. They are characterised by the property of combining with acids to form salts, and many have the power of giving an alkaline reaction with vegetable colours. All the natural alkaloids contain nitrogen as an essential constituent, and they are especially marked by possessing great medicinal power. Many artificial alkaloids have been made of recent years in which phosphorus, arsenic, and antimony occupy the place of nitrogen. For the individual properties, tests, &c., of different alkaloids, see Chemistry.