38 RESOURCES OF TENNESSEE. 



detected at a distance of some miles. They attacked the paint 

 on the Government boats and turned it to a dirty color because 

 the lead carbonate of the paint was changed to a sulphide. 

 These fumes also killed the leaves on the trees nearby. A thin 

 coating of sulphur was formed on the surface of the shale 

 dump. Considerable melanterite or copperas has also been 

 formed in the burned portion of the dump. 



In order to explain the cause of this phenomenon a little 

 physical chemistry must be considered. 



Oxidation in the strict sense of the term is a chemical change 

 in a substance whereby the substance unites with oxygen, 

 which generally comes from the air. This is the change by 

 which iron is converted into iron rust, and wood, coal, and 

 other carbonaceous materials are converted into carbon 

 dioxide, when they burn. All combustible substances under 

 ordinary conditions are constantly being oxidized at the surface 

 and when oxidation takes place, heat is always formed. But 

 the oxidation is so slow that the heat formed is radiated and 

 lost as fast as it is formed. When heat is applied to the sub- 

 stance, the rate of oxidation is increased. When the oxidation 

 is so rapid that light and heat, perceptible to the senses, are 

 evolved, the substance is said to burn or undergo combustion. 

 Combustion is, therefore, rapid oxidation. 



When a substance is oxidized at the natural, slow, gradual 

 rate and when conditions are such, that the heat produced by 

 this oxidation is not radiated and lost as fast as produced, the 

 heat accumulates and thus increases the rate of oxidation. This 

 in turn causes more heat to be formed; which again causes 

 more oxidation ; and in this way, the oxidation of the substance 

 finally becomes so rapid as to cause the substance to burn. 

 When all this takes place without external aid, we have spon- 

 taneous combustion. 



Oxidation takes place more rapidly in the presence of mois- 

 ture, or in a moist atmosphere than under dry conditions. In 

 fact oxidation is so slow in a dry atmosphere that it may hardly 

 be noticed. Oxygen in large quantities is found dissolved in 

 rain water and fresh water, such as is present at flood time, as 

 a result of its fall through the air. Analyses of rain water show 

 that about 30 per cent of the dissolved gases is oxygen. Thus 



