Friday, February 29, 2008
Poison
Poisons are usually not used for their toxicity, but may be used for their other properties. The property of toxicity itself has limited applications: mainly for scheming pests and weeds, and for preserving building materials and food stuffs. Where possible, specific agents which are less noxious to humans have come to be preferred, but exceptions such as phosphine continue in use.
Throughout human history, premeditated application of poison has been used as a method of elimination, murder, suicide and execution. As a method of execution, poison has been ingested, as the ancient Athenians did, inhaled, as with carbon monoxide or hydrogen cyanide, or injected. Many languages describe lethal injection with their corresponding words for "poison shot". Poison was also employed in concentrate warfare. For example, the 14th century Chinese text of the Huo Long Jing written by Jiao Yu outlined the use of a poisonous gunpowder mixture to fill cast iron grenade bombs.
Poisonous materials are often used for their chemical or physical properties other than being poisonous. The most effective, easiest, safest, or cheapest option for use in a chemical production may be a poisonous material. Particularly in experimental laboratory syntheses a specific reactivity is used, despite the toxicity of the reagent. Chromic acid is an example of such a "simple to use" reagent. Many technical applications call for some specific physical properties; a toxic essence may possess these properties and therefore be superior. Reactivity, in particular, is important. Hydrogen fluoride (HF), for example, is poisonous and exceptionally corrosive. However, it has a high affinity for silicon, which is exploited by using HF to etch glass or to manufacture silicon semiconductor chips.
Throughout human history, premeditated application of poison has been used as a method of elimination, murder, suicide and execution. As a method of execution, poison has been ingested, as the ancient Athenians did, inhaled, as with carbon monoxide or hydrogen cyanide, or injected. Many languages describe lethal injection with their corresponding words for "poison shot". Poison was also employed in concentrate warfare. For example, the 14th century Chinese text of the Huo Long Jing written by Jiao Yu outlined the use of a poisonous gunpowder mixture to fill cast iron grenade bombs.
Poisonous materials are often used for their chemical or physical properties other than being poisonous. The most effective, easiest, safest, or cheapest option for use in a chemical production may be a poisonous material. Particularly in experimental laboratory syntheses a specific reactivity is used, despite the toxicity of the reagent. Chromic acid is an example of such a "simple to use" reagent. Many technical applications call for some specific physical properties; a toxic essence may possess these properties and therefore be superior. Reactivity, in particular, is important. Hydrogen fluoride (HF), for example, is poisonous and exceptionally corrosive. However, it has a high affinity for silicon, which is exploited by using HF to etch glass or to manufacture silicon semiconductor chips.
Tuesday, February 12, 2008
Eye
Eyes are organs that spot light. Different kinds of light-sensitive organs are found in a variety of animals. The simplest eyes do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms but can hardly be called vision. More complex eyes can decide shapes and colors. The visual fields of some such complex eyes largely overlap, to allow better depth sensitivity, as in humans; and others are placed so as to diminish the overlap, such as in rabbits and chameleons.
In the human eye, light enters the pupil and is focused on the retina by the lens. Light-sensitive nerve cells called rods (for brightness) and cones (for color) react to the light. They interact with each other and send messages to the brain that indicate brightness, color, and contour.
In most vertebrates and some mollusks, the eye works by allowing light to enter it and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye. The cone cells (for colour) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals. The visual signals are then transmitted to the brain via the optic guts. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris which regulates the passion of the light that enters the eye. The eyes of cephalopods, fish, amphibians and snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens—similar to how a camera focuses.
In the human eye, light enters the pupil and is focused on the retina by the lens. Light-sensitive nerve cells called rods (for brightness) and cones (for color) react to the light. They interact with each other and send messages to the brain that indicate brightness, color, and contour.
In most vertebrates and some mollusks, the eye works by allowing light to enter it and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye. The cone cells (for colour) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals. The visual signals are then transmitted to the brain via the optic guts. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris which regulates the passion of the light that enters the eye. The eyes of cephalopods, fish, amphibians and snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens—similar to how a camera focuses.
