Glossary 617 



Atom: The smallest unit of matter into which an element can be divided 



and retain its identity. 

 Atomic theory (d-tom'ik) : The theory that all matter is composed of 



atoms. 



Attraction: A power in a body by which it draws things to itself. 

 Auditory canal (6'di-to-ri) : The passage extending inward from the 



outer ear to the tympanic membrane, or ear drum. 

 Auditory nerve: The nerve which carries sound impulses from the 



cochlea, in the inner ear, to the brain. 



Auricle (6'ri-k'l) : One of the two upper chambers of the heart. 

 Autogiro (6-to-ji'ro) : A heavier-than-air machine, capable of flying in 



air, and supported by a horizontal wheel, or "windmill." 



B 



Bacilli (bo-silt) : Rod-shaped bacteria, such as tuberculosis germs. 

 Bacteria (bak-te'ri-d) : The plural of bacterium. The smallest form of 



plant life; consists of a single cell. 



Bacteriology (bak-te-ri-ol'o-ji) : The science which treats of bacteria. 

 Balanced aquarium: An aquarium in which the materials needed for 



the continued life of both animals and plants are mutually provided. 

 Balanced terrarium (te-ra'ri-wm) : A box containing earth, in which 



land animals and land plants supply each other with the materials 



needed for continued life. 

 Balloon: A bag made of some light material and filled with gas or 



heated air so that it will rise and float in the atmosphere. 

 Barograph (bar'6-graf) : An instrument for recording automatically the 



variations of atmospheric pressure. 

 Barometer (bd-rom'e-ter) : An instrument for determining the pressure 



or weight of the atmosphere ; commonly used to forecast the weather. 

 Base : An alkaline substance ; a substance which will combine with an 



acid to form a salt. 



Battery: A group of cells for generating electricity. 

 Beaker: An open-mouthed, thin glass vessel for use in laboratories. 

 Beam: A group of parallel rays of light. 

 Belt of equatorial calms: A hot and almost windless section of the 



f earth near the equator; the heat equator. 



Biceps (bi'seps) : The muscle of the upper arm that bends the forearm. 

 Bichloride of mercury (bi-klo'rid) : A powerful germicide containing 



chlorine and mercury. 

 Bicuspid (bi-kus'pid) : A tooth with a blunt crown, adapted for crush- 



e ing and grinding food. 

 Bile (bil) : A secretion of the liver which aids in digestion and carries 



away poison from the liver. 



Biplane: An airplane with two pairs of wings, one above the other. 

 Bituminous coal (bi-tu'mi-nws) : A soft variety of natural coal that 



produces much smoke in burning. 

 Blade: The broad, flat part of a leaf. 

 Blast furnace: A large steel container lined with fire brick; used for 



smelting ore. 

 Block and tackle: A device, consisting of one or more movable pulleys 



combined with one or more fixed pulleys, for moving weights with a 



small amount of effort. 

 Blood: The fluid which flows through the heart, arteries, and veins of 



the body. It consists of a colorless liquid, plasma, containing red cor- 



