Absorbing duct - tube used in a ventilator system for attenuating sound waves while offering low resistance to a continuous flow of air.

Absorption bands - dark bands or liens present in spectrum as a result of the absorption by an intervening medium. Atoms usually absorb energy of only a single wavelength, thus producing lines, such as the sodium D line.

Absorption coefficient - (acoustics) when a surface is exposed to a field of sound, it refers to the ratio of the sound energy absorbed by the surface to the total sound energy that strikes it. An absorption coefficient of 1 would imply that all of the sound energy was absorbed.
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Absorption tube - a tube filled With a absorbent (generally finely divided solid or paste). It is used to absorb gases and vapours.

Above-ground tank - device that is situated in such a way that the entire surface area of the tank is completely above the plane of the adjacent surrounding surface and the entire surface area of the tank (including the tank bottom) is able to be visually inspected.

Abrasion -
1. the removal of surface material from any solid through the frictional act of another solid, liquid or a gas or combination or by chemical degradation.
2. the wearing away or rounding of surface by friction or rubbing as from action of glaciers, wind, and water borne sand on rock fragments and minerals.
It is most effective when the impact of particles on bedrock is vigorous and the particles themselves are: coarse, hard and angular and there is no vegetation to protect the rocks and soil, especially in deserts. It is effective along the scoastline too, where. it wears away at the cliffs as well as the shore.

Abstraction - a term used in water pollution, referring to the process by which a stream, by the lateral extension of its valley, takes over the catchments of neighbouring streams, thus leading to an overall reduction in number of streams. It usually occurs at the upper end of a drainage line.

Abyssal - pertaining to the zones of greatest depth at which water has more or less uniform temperature, also pertaining to the organisms of that environment.

Abyssal hill - relatively small topographic feature of the deep ocean floor which is ranging up to 1000 m high and a few 1 kilometres wide.

Abyssopelagic - refers to organisms that are I living at water depths greater than 3000 m..acacia refers to the genus of leguminous trees (also known as wattles) of tropical and subtropical origin (especially Australasian) which may form dominant vegetation in arid areas. Many commercial products are obtained from acacias, including dyes, perfumes, timber etc.

Acanthodii - refers to a group of extinct fishes which were originating in the Silurian Period. They were small fishes, having fins supported by large spines and were the first vertebrates known to have possessed jaws.

Acarina - mites and ticks which are small Arachnid a with rounded bodies. Mites are very abundant in the soil, feeding on plant material and invertebrates.

Acarophily - a symbiotic relationship between plants and mites.

Accelerated stabilisation - in composing, this desired condition may be achieved by more frequent turning of the windows, or by raising the temperature as the material is undergoing combination and fermentation before being set out in windows. In sludge treatment, accelerated stabilisation is achieved by use of the activated sludge treatment, by raising the temperature of the sludge itself etc.

Accessory mineral - a mineral which is found in small amounts in a rock and disregarded in the classification of that rock (which is based on essential minerals). Accessory minerals can provide evidence about the origin of the rock (e.g., the presence of metamorphic minerals in a sandstone reveals a provenance, at least in part, from a metamorphic belt).

Acclimated microorganisms - bacteria, fungi and other microscopic organisms that can adjust themselves to changes in the environment, such as temperature, oxygen levels and pollution levels.

Accretion - the attachment of airborne material to fixed or falling or flying objects. Ice accretion is found on wires, hailstones or aircraft wings when the air has super cooled could droplets, drizzle or rain, and is particularly dangerous on the rigging of ships in Polar Regions or on TV masts on hills in winter. Pollution accretion has been exemplified by smoke deposition on window frames, ventilation intakes etc., where the airflow is swift and curved.

Acetylcholine - substance released in minute amounts when impulses arrive at many nerve endings, so 'passing' on the nerve impulses to other nerve cells or effectors (e.g., muscles). Its effects disappear rapidly after secretion because it is destroyed by the enemy.

Acetylene - a colouress, unsaturated, highly flammable, aliphatic hydrocarbon, existing as gas at room temperature. It is obtained by cracking petroleum hydrocarbons, or natural gas by partial oxidation, or by the action of water on calcium carbide. It is used in the manufacture of a number of chemicals such as vinyl chloride, vinylidene chloride, vinyl acetate, acrylonitrile, acetadehyde, acrylates, trichlorethylene, 1,4-butanediol and carbon black.
It is also used for cutting, welding, brazing and localised hardening of glass and metals. The. gas or vapours, when present in high concentrations in air, and act primarily as simple asphyxiant without other significant physiological effects but heavy concern transitions have anaesthetic properties and may render a person unconscious