_Energy._ Ability to do work or change the nature of bodies. Heat and light are considered forms of energy. _Entomology._ The study of insects. _Environment._ Surrounding conditions, influences, and powers of all sorts. Everything which, directly or indirectly, affects the fate of an organism forms part of its environment. _Eocene._ Relating to the lowest division of the Tertiary strata. _Epiphyte._ A plant which grows upon another but which is not fed by it. A vegetable parasite on an animal body. _Equation._ A compensation for inaccuracy. In mathematics, a formula affirming the equivalence of two expressions. An expression in symbols of a chemical reaction. _Equinox._ The time when the sun crosses the equator, and day and night are of equal length. _Erbium._ A metallic element. Symbol: Er. _Erg._ The work done by a force that, when it acts for one second on mass of one gram, produces a velocity of one centimeter per second. _Erosion._ Gradual destruction or wearing away, as of land by water. _Esthetics._ Also spelled: æsthetics. The study of beauty. It is, at present, rather an art or a branch of philosophy than an exact science. _Ether._ An anesthetic liquid. Totally distinct is this meaning: the medium through which light-waves pass, which is supposed to have a density infinitely less than that of the lightest gas. The nature of the ether is in dispute. _Ethics._ The branch of philosophy concerned with human conduct. It may at some future time rank as a science. _Ethnology._ The science dealing with the races of mankind. It has not, alas, always been thoroughly scientific. _Ethyl alcohol._ The alcohol which cheers and also inebriates men. Before Prohibition, practically the only kind of alcohol which people drank. _Europium._ A metallic element. Symbol: Eu. _Evaporate._ Change (usually with no great rapidity) from the liquid to the gaseous state. _Evergreen._ A tree or shrub which has green leaves in all seasons. _Evolution._ Development, change. In biology, the development of a race or species by gradual change from another type. The Theory of Evolution contradicts the notion that every type was originally created in the form which it now has. _Experiment._ An event (or, in the case of the verb: to bring about an event) designed for observation, in order that an unknown fact or principle may be brought to light, or simply to instruct. _Explain._ To reduce a phenomenon to the terms of a general principle. To bring into a system. _Fact._ Something which is true. If only a single investigator has reported a certain circumstance or condition, it is not likely to be considered a fact by scientists. _Factor._ A substance which takes part in a chemical reaction. _Facula._ A bright spot or streak on the sun.