150 FIRST YEAR COURSE IN GENERAL SCIENCE 



162. Chlorine. Chlorine is of interest principally be- 

 cause of the importance of some of its compounds. The ele- 

 ment is a greenish-yellow gas of disagreeable odor; when 

 breathed, it is irritating and suffocating. It does not exist 

 free in nature, but in combination with other elements it is 

 found in many minerals. The most abundant compound of 

 chlorine is common salt, which is found both in the ocean 

 and as a solid mineral in the earth. Chlorine can be pre- 

 pared from common salt for use in bleaching or in disin- 

 fecting. A powder called chloride of lime is commonly used 

 for these purposes, as it gives off its chlorine very readily. 



163. Carbon. Most of the solid elements are metals, 

 but there are three common solid elements carbon, sul- 

 phur, and phosphorus which are not metals. 



Carbon is black, except in the case of the diamond, which 

 is usually colorless. There are several other varieties of the 

 element carbon: graphite (called black lead), lampblack or 

 soot, coke, and charcoal. Mineral coal is an impure form 

 of carbon. 



All forms of carbon are combustible, but they must have 

 a comparatively high temperature in order to burn. The 

 kindling point of charcoal is lowest, and that of graphite is 

 highest. When charcoal and diamond are burned in pure 

 oxygen, they both form the same product, carbon dioxide. 

 This proves that, unlike as they are in appearance and 

 properties, charcoal and diamond are the same element. 



164. Coal and Charcoal. Many, many thousands of 

 years ago areas of land overgrown with great forests slowly 

 sank and were covered with water. The fallen trees were 

 buried under layers of sand and mud which settled in the 

 water. The plant material, shut away from the air, oxidized 

 very slowly. In the process of slow combustion, gases were 

 formed and passed away, leaving beds of solid matter, 

 mostly carbon, which is called coal. Coal is the residue of 

 incomplete combustion of wood and other vegetable matter. 



