638 Our Surroundings 



Refrigeration: The preserving of foods by keeping them at a low tem- 

 perature, thus hindering the growth of molds and bacteria. 



Relative humidity (hu-mid'i-ti) : The ratio between the amount of 

 moisture in the air at a given time and temperature and the maximum 

 amount it could hold at that temperature; usually expressed in terms 

 of percentage. 



Relay (re-la') : A device for increasing the strength of the electric cur- 

 rent which operates the sounder in a telegraph circuit. 



Rennin: A ferment in gastric juice which coagulates milk. 



Reproduce: To form similar matter. 



Reservoir: A place where water is stored for use. 



Resistance: Opposition, as that offered by a wire to the passage of an 

 electric current. 



Respiration: The taking in of oxygen by a plant or animal, the com- 

 bining of this oxygen with digested food material, and the excretion 

 of carbon dioxide. 



Respiratory (re-spir'd-to-ri) : Pertaining to breathing. 



Retina (ret'i-nd) : The inner coating of the eye which receives pictures 

 of objects, called images, and transmits them through the optic nerve 

 to the brain. 



Retort: A closed, air-tight chamber in which wood, coal, and other sub- 

 stances are subjected to heat to make charcoal, coke, gas and other 

 products. 



Revolution: The act of passing around a body in a regular orbit. 



Ribs: A series of twelve pairs of curved bones, which serve to protect 

 the chest cavity. 



Roasting: Cooking by exposing to continued high temperature, as in 

 an oven. 



Rock: The inorganic material which forms the solid part of the earth. 



Root: The underground part of a plant. 



Root hair: A tiny offshoot of a root where absorption takes place. 



Rotation: The act of turning around on an axis or center. 



Rotation of crops: Raising of a series of different crops on the same 

 land in different years. 



Rust: Iron which has oxidized. 



Saccharin (sak'd-rin) : An intensely sweet drug obtained from coal tar; 



often used as an adulterant for sugar. 



Saliva: The secretion of the salivary glands; aids digestion. 

 Salt: A neutral substance formed by the action of an acid with an 



alkali. A seasoning for food. 

 Sand: A loose material consisting of fine particles of rock. Used in 



glass making and concrete. 

 Sandstone: A sedimentary rock consisting of sand grains held together 



by a material containing clay or silica ; used for building. 

 Sanitary: In a healthful condition; free from germs. 

 Sanitation: The application of measures for promoting and protecting 



health. 

 Saprophyte (sap'ro-fit) : A plant, such as a mushroom or an Indian 



pipe, which lives on dead and decaying vegetable matter. 

 Satellite (sat'^-llt) : A small planet revolving about a larger one; a 



moon. 

 Saturate (sat'u-rat) : To soak. 



