Heat is the energy that flows between two objects because of a difference in temperature. Heat always flows from a body at a higher temperature to one at a lower temperature.


Hibernation is a state of inactivity, in which an animal's heart rate, body temperature, and breathing rate are decreased in order to conserve energy through the cold months of winter. A similar state, known as estivation, occurs in some desert animals during the dry months of summer.

Hologram and Holography

Holography is a method of producing a three-dimensional (3-D) image of an object. (The three dimensions are height, width, and depth.) The image it brings to life is referred to as a hologram, from the Greek word meaning "whole picture." Unlike a two-dimensional picture, a hologram allows a person to look "around" and "behind" its subject.


Hormones are chemicals produced by one kind of tissue in an organism and then transported to other tissues in the organism, where they produce some kind of response. Because of the way they operate, hormones are sometimes called "chemical messengers." Hormones are very different from each other—depending on the functions they perform—and they occur in both plants and animals.


Horticulture is the science and art of growing and caring for plants, especially flowers, fruits, and vegetables. Whereas agronomy (a branch of agriculture) refers to the growing of field crops, horticulture refers to small-scale gardening.

Human Evolution

Since the mid-nineteenth century, Western scientific thought has stated that all present-day species on Earth, including man, have arisen from earlier, simpler forms of life. This theory means that the story of human evolution begins with a creature most people today would not consider human.

Human Genome Project

The Human Genome Project is the scientific research effort to construct a complete map of all of the genes carried in human chromosomes. The finished blueprint of human genetic information will serve as a basic reference for research in human biology and will provide insights into the genetic basis of human disease.


Hydrogen is the simplest of all chemical elements. It is a colorless, odorless, tasteless gas that burns in air to produce water.

Hydrologic Cycle

Hydrologic cycle is the phrase used to describe the continuous circulation of water as it falls from the atmosphere to Earth's surface in the form of precipitation, circulates over and through Earth's surface, then evaporates back to the atmosphere in the form of water vapor to begin the cycle again. The scientific field concerned with the hydrologic cycle, the physical and chemical properties of bodies of water, and the interaction between the waters and other parts of the environment is known as hydrology.

Ice Ages

Ice ages were periods in Earth's history when glaciers and vast ice sheets covered large portions of Earth's surface. Earth's average annual temperature varies constantly from year to year, from decade to decade, and from century to century.


An iceberg is a large mass of free-floating ice that has broken away from a glacier. (Glaciers are flowing masses of ice, created by years of snowfall and cold temperatures.) Beautiful and dangerous, icebergs are carried about the ocean surface until they melt.

Imaginary Number

An imaginary number is the square root of a negative real number. (The square root of a number is a second number that, when multiplied by itself, equals the first number.) As an example, √−25 is an imaginary number.

Immune System

The immune system in a vertebrate (an organism with a backbone) consists of all the cells and tissues that recognize and defend the body against foreign chemicals and organisms. For example, suppose that you receive a cut in your skin.

Incandescent Light

Incandescent light is given off when an object is heated until it glows. To emit white light, an object must be heated to at least 1,341°F (727°C).

Indicator Species

Indicator species are plants and animals that, by their presence, abundance, or chemical composition, demonstrate some distinctive aspect of the character or quality of the environment. For example, in areas where metal-rich minerals can be found at the soil surface, indicator species of plants accumulate large concentrations of those minerals in their tissues.

Industrial Minerals

The term industrial minerals is used to describe naturally occurring nonmetallic minerals that are used extensively in a variety of industrial operations. Some of the minerals commonly included in this category include asbestos, barite, boron compounds, clays, corundum, feldspar, fluorspar, phosphates, potassium salts, sodium chloride, and sulfur.

Industrial Revolution

Industrial Revolution is the name given to changes that took place in Great Britain during the period from roughly 1730 to 1850. It was originated by German author Friedrich Engels (1820–1895) in 1844.

Infrared Astronomy

Infrared astronomy involves the use of special telescopes that detect electromagnetic radiation (radiation that transmits energy through the inter-action of electricity and magnetism) at infrared wavelengths. The recent development of this technology has led to the discovery of many new stars, galaxies, asteroids, and quasars.


Insects are invertebrates in the class Insecta, which contains 28 living orders. The animals that make up this class have a number of distinctive features.

Integrated Circuit

An integrated circuit is a single, miniature circuit with many electronically connected components etched onto a small piece of silicon or some other semiconductive material. (A semiconductor is a nonmetallic material that can conduct an electric current, but does so rather poorly.) Integrated circuits are more commonly known as microchips.

Integumentary System

The human integumentary system is made up of the skin, hair, nails, and associated glands. Its main function is to protect the body.


Interference is the interaction of two or more waves. Wave motion is a common phenomenon in everyday life.


Interferometry is the process of making measurements by allowing sound, light, or other kinds of waves to interfere with each other. Interferometry is used for a large variety of purposes, such as studying the velocity of sound in a fluid, locating the position and properties of objects in space, determining the size and properties of objects without actually touching or otherwise disturbing them, and visualizing processes such as crystal growth, combustion (burning), diffusion (spreading), and shock wave motion.

Internal-Combustion Engine

The invention and development of the internal-combustion engine in the nineteenth century has had a profound impact on human life. The internal-combustion engine offers a relatively small, lightweight source for the amount of power it produces.

International Ultraviolet Explorer

Developed during the 1970s, the International Ultraviolet Explorer (IUE) was a joint project between the National Aeronautics and Space Administration (NASA); Particle Physics and Astronomy Research Council (PPARC), formerly known as the Science and Engineering Research Council of the United Kingdom (SERC); and the European Space Agency (ESA). The IUE was built to explore astronomical objects such as stars, comets, galaxies, and supernovae that exist in the ultraviolet portion of space.


The Internet is a vast network that connects many independent networks and links computers at different locations. It enables computer users throughout the world to communicate and to share information in a variety of ways.

Interstellar Matter

The interstellar medium—the space between the stars—consists of nearly empty space. It is the vacuum of the universe.


Invertebrates are animals without backbones. This simple definition hides the tremendous diversity found within this group which includes protozoa (single-celled animals), corals, sponges, sea urchins, starfish, sand dollars, worms, snails, clams, spiders, crabs, and insects.