_Diesel engine._ An internal combustion motor which compresses the air highly and which successfully burns oil which is unsuitable for the ordinary motor of an automobile. _Disease._ Malady. Of some foods and drinks, an impairment in quality caused by bacteria. _Dissipation of energy._ The change of energy to such a form that it cannot be used to do work. _Dissociate._ Ionize, decompose, split up. _Doldrums._ A region of calms and light winds near the equator. _Dolichocephalic._ Long-headed, having a skull with the breadth less than four-fifths of the length. _Drug._ A medicine, a medicinal agent, especially one of vegetable origin. _Dynamics._ The branch of physics dealing with forces. _Dyne._ The amount of force that acts for a single second on a mass of one gram to give it a velocity of one centimeter per second. _Eccentric._ Not concentric to a given circle. Arranged to change rotary motion back and forth. _Echinoderm._ One of a class of animals including the sea-urchins. _Eclipse._ Of a heavenly body, obscured by passing between it and the observer or its source of light. _Ecliptic._ The apparent orbit of the sun. _Ecology._ The science dealing with the relations between organisms and their environment. _Economics._ The science of wealth, especially of its production, distribution, and consumption. _Edentate._ Toothless or without canine teeth. _Effect._ See Cause. _Efferent._ Carrying outward or away. _Efficiency._ The ratio between the amount of work put into a machine and the amount derived from it. _Efflorescence._ The loss of water of crystallization. _Electricity._ A peculiar condition of the molecules of a body or of the ether surrounding the molecules, or else a sort of fluid or other matter in the form of small bodies called electrons. It is easier to tell what electricity does or how it is produced than to define it, especially since physicists have not been able to agree about its precise nature. _Electrode._ Either pole of a cell. _Electrolysis._ The decomposition of a compound by means of an electric current. _Electrolyte._ A substance which, in solution, can carry an electric current to an appreciable extent. _Electron._ The electric charge of an atom. A very small particle which is electrically charged. The size of the electron has been variously calculated, and some physicists declare that the negative electron has only 1/1700 the mass of a hydrogen atom. _Element._ One of a number of substances formerly considered to be indivisible. An electric cell. _Emanation._ A gaseous substance produced by a radioactive material. _Embryo._ The offspring of an animal before birth or emergence from the egg. The scientific study of the embryo is called embryology. _Endogenous._ Growing from within. _Endosperm._ The nutritive tissue enclosed with the embryo in seeds. _Endothelium._ The layer of cells lining blood vessels, the interior of the heart, etc.