Photochemistry

Photochemistry, study of chemical processes that are accompanied by or catalyzed by the emission or absorption of visible light or ultraviolet radiation. A molecule in its ground (unexcited) state can absorb a quantum of light energy, or photon, and go to a higher-energy state, or excited state (see quantum theory). Such a molecule is then much more reactive than a ground-state molecule and can undergo entirely different reactions than the more stable molecule, following several different reaction pathways. One possibility is that it can simply emit the absorbed light and fall back to the ground state. This process, called chemiluminescence, is illustrated by various glow-in-the-dark objects. Another possibility is for the molecule to take part in a photo-induced chemical reaction; it may break apart (photodissociate), rearrange, isomerize, dimerize, eliminate or add small molecules, or even transfer its energy to another molecule. Photochromic compounds—compounds that change color reversibly in going from the dark to the light—are generally compounds that are capable of reversible isomerization, or rearrangement. In the absence of light, the compound exists in its most stable form, which exhibits a particular color; in the presence of light, the compound goes to a less stable form, which exhibits a different color. After removal of the light, the compound will revert back to its original state. The best-known and most important photochemical reaction is photosynthesis, the complex, chlorophyll-catalyzed synthesis of sugars from carbon dioxide and water in the presence of light. Other extremely important and complex photochemical reactions take place in the eye. Photochemistry is indispensible to industries involved with dyes, photography, television, and many other applications of light and color.

Aerosol dispenser

Aerosol dispenser, device designed to produce a fine spray of liquid or solid particles that can be suspended in a gas such as the atmosphere. The dispenser commonly consists of a container that holds under pressure the substance to be dispersed (e.g., paints, insecticides, medications, and hair sprays) and a liquefied gas propellant. When a valve is released, the propellant forces the substance through an atomizer and out of the dispenser in the form of a fine spray. These devices are more properly termed spray dispensers rather than aerosol dispensers because the particles of the dispersed substance are usually larger than the particles of a true aerosol (see colloid), such as a fog or a smoke. Freon was the most common aerosol propellant, but its use has been banned because it is believed to contribute to destruction of the ozone layer of the stratosphere; common propellants now include propane, butane, and other hydrocarbons.

chronometer

Chronometer, mechanical timekeeping device of great accuracy, particularly one used for determining longitude (see latitude and longitude) at sea. Early weight- and pendulum-driven clocks were inaccurate because of friction and temperature changes and could not be used at sea because of the ship's motion. In 1735 John Harrison invented and constructed the first of four practical marine timekeepers. The modern marine chronometer is suspended to remain horizontal whatever the inclination of the ship and differs in parts of its mechanism from the ordinary watch. A chronometer may provide timekeeping accurate to within 0.1 second per day. See also Ferdinand Berthoud.

Electrocardiography

Electrocardiography (ĭlĕk'trōkärdēŏg`rəfē), science of recording and interpreting the electrical activity that precedes and is a measure of the action of heart muscles. Since 1887, when Augustus Waller demonstrated the possibility of measuring such action, physicians and physiologists have recorded it in order to study the heart's normal behavior and to provide a method for diagnosing abnormalities. Electrical current associated with contraction of the heart muscles passes through the various tissues and reaches the surface of the body. What is actually recorded is the change in electrical potential on the body surface. The first practical device for recording the activity of the heart was the string galvanometer developed by William Einthoven in 1903. In this device a fine quartz string is suspended vertically between the poles of a magnet. The string is deflected in response to changes in electrical potential and its movement can be optically enlarged and photographed, or, if an immediately visible record is desired, the string's movement can be recorded on a sheet of paper. A more sophisticated form of the electrocardiograph employs a vacuum-tube amplifier. The greatly amplified current from the body deflects a mirror galvanometer that causes a beam of light to move across a light-sensitive film. When an electrocardiograph is taken, electrodes (leads) are attached to the extremities and to the left chest. The recordings obtained in this manner are called electrocardiograms, or more simply EKG's or ECG's. A normal EKG shows a sequence of three waves arbitrarily labeled P, QRS, and T. The P wave is a small, low-amplitude wave produced by the excitation of the atria of the heart. It is followed by a resting interval that marks the passage of electrical impulses into the ventricles. Following this interval comes the QRS wave, a rapid, high-amplitude wave marking ventricular excitation, and then a slow-building T wave denoting ventricular recovery. Abnormalities may be noted from deviation in wave form, height, direction, or duration. The type of abnormal wave may sometimes indicate the type of heart disorder. Usually the physician must associate the EKG with other clinical observations to determine the cause of the abnormality.

nutation

Nutation, in astronomy, a slight wobbling motion of the earth's axis. The causes of nutation are similar to those of the precession of the equinoxes, involving the varying attraction of the moon on the earth's equatorial bulge. However, the period of the motion is only 18.6 years, the same as that of the precession of the moon's nodes, as opposed to the nearly 26,000-year period of the precession of the equinoxes. Nutation was discovered by the English astronomer James Bradley in 1728 but was not explained until 20 years later.