640 Our Surroundings 



Skin: The external covering of the body. It is composed of two layers, 



the dermis and the epidermis. 

 Slag: A substance formed by a combination of lime and impurities from 



ore during smelting. Used in the manufacture of cement. 

 Sleet: Snow and rain falling together. 



Smelting: The process of melting or fusing by which metals are ex- 

 tracted from their ores. 

 Snow: Tiny crystals of ice, formed by the freezing of water vapor in 



the air. 

 Soap: A cleansing agent, usually made by combining an alkali with fat 



or oil. 

 Soil: A substance composed of inorganic matter formed by the breaking 



up of rocks and of organic matter derived from plants and animals. 



Necessary for the growth of plant life. 

 Soil exhaustion: A condition of the soil caused by failure to restore 



to it the nutrients which growing plants have taken from it. 

 Solar day (so'ldr) : The length of time which elapses between successive 



appearances of the sun at its highest point in the heavens. 

 Solar eclipse: An eclipse of the sun. 

 Solar system: The sun with the group of planets which revolve 



around it. 



Solar time : Actual time as determined by the sun. 

 Solid: A substance that will keep its shape without a container to hold 



it in place.^ 



Soluble (sol'u-b'l) : Capable of being dissolved, as in a liquid. 

 Solution (sp-lu'shwn) : A complete and perfect mixture of a solid, liquid 



or gas with a liquid. A mixture of salt and water is a solution. 

 Solvent (sol'wnt) : A liquid capable of dissolving substances. 

 Sound: A sensation due to the action of air waves upon the ear. 

 Sounder: The receiving instrument of a telegraph circuit. It gives out 



a series of clicks as the charged electric circuit is completed and broken. 

 Special property: A quality which shows itself in one or more kinds 



of^ matter, but not in all kinds. 

 Species (spe'shez) : A group of animals or plants having one or more 



common characteristics which distinguish them from other similar 



groups. 

 Spectroscope: An instrument for breaking up light to study the colors 



of which it consists. 

 Spectrum: A band of seven colors red, orange, yellow, green, blue, 



indigo, and violet formed when white light is broken up, as when 



passed through a triangular glass prism. 

 Spinal cord: The cord of nerve tissue connecting with the brain and 



running lengthwise through the rings of the backbone. The trunk 



line through which the brain makes nerve connections with all parts 



of the body. 

 Spiracles (spir'd-k'lz) : Minute air holes in the sides of insects, through 



which they breathe. 

 Spleen: A ductless gland near the lower wall of the stomach, whose use 



is not fully understood. 



Spontaneous combustion (spon-ta'ne-ws) : A fire starting without out- 

 side aid, as the bursting into flame of an oily rag due to accumulation 



of heat from gradual oxidation. 

 Spores: Bacteria which have developed thick cell walls. These preserve 



them, inactive but alive, through long periods when conditions are un- 

 favorable for active life. 



