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.

The Elements through the Ages

Some elements have been known since antiquity. Gold ornaments from the Neolithic period have been discovered. Gold, iron, copper, lead, silver, and tin were used in Egypt and Mesopotamia before 3000 B.C. However, recognition of these metals as chemical elements did not occur until modern times.

Greek Concept of the Elements

The Greek philosophers proposed that there are basic substances from which all things are made. Empedocles proposed four basic "roots," earth, air, fire, and water, and two forces, harmony and discord, joining and separating them. Plato called the roots stoicheia (elements). He thought that they assume geometric forms and are made up of some more basic but undefined matter. A different theory, that of Leucippus and his followers, held that all matter is made up of tiny indivisible particles (atomos).

This theory was rejected by Aristotle, who expanded on Plato's theory. Aristotle believed that different forms (eidos) were assumed by a basic material, which he called hulé. The hulé had four basic properties, hotness, coldness, dryness, and moistness. The four elements differ in their embodiment of these properties; fire is hot and dry, earth cold and dry, water cold and moist, and air hot and moist. Although Aristotle proposed that an element is "one of those simple bodies into which other bodies can be decomposed and which itself is not capable of being divided into others," he thought the metals to be made of water, and called mercury "silver water" (chutos arguros). His idea that matter was a single basic substance that assumed different forms led to attempts by the alchemists to transmute other metals into gold.

Evolution of Modern Concepts

Although much early work was done in chemistry, especially with metals, and many recipes were recorded, there were few developments in the conception of the elements. In the 16th cent. Paracelsus proposed salt, mercury, and sulfur as three "principles" of which bodies were made, although he apparently also believed in the four "elements." Van Helmont (c.1600) rejected the four elements and three principles, substituting two elements, air and water.

Robert Boyle rejected these early theories and proposed a definition of chemical elements that led to the currently accepted definition. His definition is strikingly similar to Aristotle's earlier definition. In The Sceptical Chymist (1661) Boyle wrote, "I now mean by elements … certain primitive and simple, or perfectly unmingled bodies; which not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixed bodies [chemical compounds] are immediately compounded, and into which they are ultimately resolved."

Whereas Aristotle and other early philosophers tried to determine the identity of the elements solely by reason, Boyle and later scientists used the results of numerous experiments to identify the elements. In 1789 Antoine Lavoisier published a list of chemical elements based on Boyle's definition; this encouraged adoption of standard names for the elements. Although some of his elements are now known to be compounds, such as metallic oxides and salts, they were at the time accepted as elements since they could not be decomposed by any method then known.

In 1803 John Dalton proposed (as part of his atomic theory) that all atoms of an element have identical properties (including mass), that these atoms are unchanged by chemical action, and that atoms of different elements react with one another in simple proportions. Although symbols for some of the elements already existed, they were by no means universally accepted, and each compound also had a unique symbol that was unrelated to its chemical composition. Dalton devised a new set of circular symbols for the elements and used a combination of elemental symbols to represent a compound. For example, his symbol for oxygen was ○, and for hydrogen ȯ. Since he thought water contained one atom of hydrogen for every atom of oxygen, he formed the symbol for water by writing the symbols for hydrogen and oxygen touching one another, ȯ&nosp;○. J. J. Berzelius was the first to use the modern method, letting one or two letters of the element's name serve as its symbol. He also published an early table of atomic weights of 24 elements with most values very close to those now in use.

Discovery of the Elements

As noted above, some of the elements were discovered in prehistoric times but were not recognized as elements. Arsenic was discovered around 1250 by Albertus Magnus, and phosphorus was discovered about 1674 by Hennig Brand, an alchemist, who prepared it by distilling human urine. Only 12 elements were known before 1700, and only about twice that many by 1800, but by 1900 over 80 elements had been identified. In 1919 Ernest Rutherford found that hydrogen was given off when nitrogen was bombarded with alpha particles. This first transmutation encouraged further study of nuclear reactions, and eventually led to the discovery in 1937 of technetium, the first synthetic element. Neptunium (atomic number 93) was the first transuranium element to be synthesized (1940). Its discovery prompted the search that led to the discovery of other transuranium elements.

spectroscope

Spectroscope, optical instrument for producing spectral lines and measuring their wavelengths and intensities, used in spectral analysis (see spectrum). When a material is heated to incandescence it emits light that is characteristic of the atomic makeup of the material. In the original spectroscope design in the early 19th cent., light entered a slit and a collimating lens transformed the light into a thin beam of parallel rays. A prism then separated the beam into its spectrum. The observer then viewed the spectrum through a tube with a scale that was transposed up the spectrum image, enabling its direct measurement. With the development of photographic film, the more accurate spectrograph was developed. It was based on the same principle as the spectroscope, but it had a camera in place of the telescope. In recent years the electronic circuits built around the photomultiplier tube have replaced the camera, allowing real-time spectrographic analysis of far greater accuracy. Such spectrum analysis, or spectroscopy, has become an important scientific tool for analyzing the composition of unknown material. It has found applications in fields as disparate as astronomy and forensic chemistry.