[3] Protoplasm is not a single chemical compound; it is the name given to the soft, slimy substance of cells, and contains many chemical compounds—proteids, fats, and others; some on the way to assume greater chemical complexity; others in process of destruction. The critical highest chemical body concealed in protoplasm has no generally recognised name. It is a proteid-like body, consisting chiefly of carbon, oxygen, hydrogen, and nitrogen, with some saline constituents. This is the real ultimate “living matter,” and I suggested in the _Encyclopedia Britannica_ (article Protozoa) in 1886 that it should be called “plasmogen.” XX CHEMISTRY AND PROTOPLASM When the chemist examines living cell-substance or protoplasm—as free as possible from dead envelopes and products of its own activity—so as to make out, if he can, what it is chemically, he finds that it consists of the elements carbon, oxygen, hydrogen, and nitrogen, with some sulphur. Phosphorus and some potash, soda and lime in small quantity, are also very usually associated with the elements named. These are combined in the protoplasm so as to form chemical compounds resembling and including white of egg, and are called “proteids.” A chemical compound is a very definite and special thing, and when one says so-and-so is a definite chemical compound, one means that it is not a mere “mixture,” but is composed of chemical elements (some out of the long list of about eighty indestructible, undecomposable, “simple” bodies—gases, liquids, metallic and non-metallic solids—recognised by chemists and known as such), peculiarly united to, or “combined” with, one another in definite proportions by weight. Take, as an example, water. Water is a definite chemical compound, formed by the chemical union of two pure elements, the gases hydrogen and oxygen—eighteen ounces of water consist of two ounces of hydrogen and sixteen ounces of oxygen. At a temperature above that of boiling water the gases, when they unite, contract to form water-vapour, three pints of the uniting gases (consisting of two pints of hydrogen and one of oxygen) forming two pints only of water-vapour. This, when it is cooled to a temperature below 212 deg. Fahr., suddenly contracts to a few thimblefuls of pure liquid water. Neither oxygen nor hydrogen “uncombined” liquefy till far below zero.