_Indo-European_ or _Indo-Germanic_. Pertaining to a great family of languages including most of those spoken in Europe and by the white men in America as well as some used in India and other parts of Asia. _Inductance._ The electrification of a conductor placed near a circuit or charged body, resulting from the proximity. _Inert._ Not easily made to react. _Inertia._ The property of matter by which it tends to remain at rest if it is at rest, or, if it is in motion, to remain in motion in a straight line. Our notion of inertia may possibly be changed by Einstein’s theory. _Inflorescence._ Flowering, the arrangement of flowers on a plant. _Infra-._ In compounds, below. _Infusoria._ A class of protozoa which possess hairlike outgrowths. _Inorganic._ Not derived from living organisms. Pertaining to the chemical compounds which do not contain carbon--but carbonates and a few other carbon compounds are considered to be inorganic. _Insect._ A member of a class of small and invertebrate animals, all of which have bodies clearly divisible into head, thorax, and abdomen. _Instinct._ An action pattern present from birth. (The varying theories of instinct, especially with regard to man, are often based upon varying definitions. This is true of many of the terms here defined.) _Intelligence quotient._ The ratio between the “mental age” of an individual and the true age. The true age is often easier to determine than the mental age, that is, the age which a normal individual of a given intelligence possesses. How can intelligence be expressed numerically? Abbreviation: I. Q. _Interference._ The influence which two waves have upon each other. _Invertebrate._ Spineless, having no backbone. _Iodine._ A solid, non-metallic element. Symbol: I. _Ion._ Either of the substances that appear at the poles in electrolysis. Particle which carries electric charge. Positive ions are cations, negative ions are anions. _Iridium._ A metallic element. Symbol: Ir. _Iron._ An important metallic element. Symbol: Fe. _Isobar._ A line connecting places which have the same barometric pressure (with corrections for varying altitude) at a given time or over a given period. _Isotherm._ A line connecting places which have the same temperature at a given moment or over a stated period of time. _Joule._ A unit of work. It is equal to ten million ergs, and it is about the energy expended in one second by an electric current of one ampere in a resistance of one ohm. _Jurassic._ Of that Mesozoic period following the Triassic. _Kidney._ One of a pair of organs which secrete urine. _Kilogram._ A unit of mass in the metric system, about the mass of a cubic centimeter of water at its greatest density. _Kinetic theory of gases._ The assumption and the teaching that the molecules of gases are constantly in motion and that the space between molecules is far greater than that occupied by the molecules themselves. The velocity of gas molecules is supposed to be very great.