KEY AND INDEX 



Calcium Carbide. A compound of calcium and carbon (Ca C a ), 

 produced in the electric furnace by heating a mixture of lime and 

 coke. When brought in contact with water, a chemical recom- 

 bination is effected whereby acetylene gas (C 2 H 2 ) is produced. 

 The change is chemically expressed thus: Ca C 3 +H 3 O=C a H 2 + 

 Ca O. See "The Introduction of Acetylene Gas," Vol. VI, p. 212. 



Calotype Process. A photographic process invented by Fox 

 Talbot (q. v.) ; the prototype of modern paper-printed photo- 

 graphs, as distinguished from the metallic-surface process of 

 Daguerre. See "Talbot's Calotype Process," Vol. VIII, p. 227. 



Camera Lucida. An instrument in which, through the use of 

 prisms, light is refracted and reflected in such a way as to throw 

 the image of a landscape or other view on a ground-glass plate, 

 where its outlines may be readily sketched. 



Camera Obscura. Essentially, as its name implies, a dark 

 chamber, into which a single ray of light is admitted through a 

 pinhole aperture. The small aperture has the effect of a lens, 

 and an image of the scene outside is thrown on the wall of the 

 chamber opposite the aperture. The photographic camera is a 

 modified camera obscura, in which the light is focused by a 

 lens. It is possible to take a photograph, of rather vague out- 

 line, by using a very small aperture, without a lens. 



Candle. This familiar modified form of lamp, consisting es- 

 sentially of a wick embedded in a solid cylinder of wax, tallow, 

 or other easily liquefied, inflammable fat, was invented late in 

 the twelfth century. For centuries it was the best of lighting 

 apparatuses, and it has by no means been supplanted altogether 

 even in our day. For the general description of methods of 

 illumination, past and present, see "The Banishment of Night," 

 Vol. VI, p. 201. 



Carbonic Acid Gas. A compound of carbon and oxygen, 

 having the formula C O a . It is given off through the lungs by 

 animal organisms, and is absorbed by plants, which thus secure 

 the carbon that enters so importantly into the vegetable 

 structure. Another and still more poisonous compound of 

 carbon oxygen is known as carbonic oxide (C O). Both these 

 gases are sometimes present in mines. See "Conditions to be 

 Considered in Mining," Vol. VI, p. 247, and "The Function of 

 Respiration," Vol. IV, p. 92. 



Cathode Rays. A peculiar manifestation of energy due to 

 passing an electrical current through a vacuum tube. These 

 rays cast a shadow and are deflected by a magnet; and, by im- 



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