Protoplasm - the so-called "physical basis of life" - is a substance which possesses the properties of spontaneous movement, nutrition, excretion, and reproduction, and constitutes the simplest of living organisms. It consists essentially of hydrogen, oxygen, carbon, and nitrogen, with a certain amount of sulphur, phosphorus, potassium, and sodium; other elements are usually present in addition. Its chemical composition is therefore complex, while, as it is almost impossible to get it pure from waste food products, its quantitative analysis has not been satisfactorily determined. Its essential property is its power of movement spontaneously under certain irritation. This, however, is accompanied by a necessary oxidisation of part of its constituents. To counterbalance this and keep up the supply of unoxidised material, the protoplasm has to feed; the waste products formed by the oxidisation or absorbed with the food have also to be got rid of, which is secured by the process of excretion. If the protoplasm absorbs more food than is required to supply the waste due to movements, it increases in bulk; and when the particle of protoplasm has increased so as to be inconveniently large, it subdivides and reproduction is effected. This is usually determined by the agency of a specialised part of the protoplasm known as the "nucleus" (q.v.), but certain of the Proteomyxa (q.v.) have no such structure but have the power of reproduction. The simplest method of studying the movements, etc., of protoplasm is the observation of the white corpuscles of the blood or the common fresh-water Amoeba (q.v.), or Proteus-Animalcule.