30 EXPERIMENTAL GENERAL SCIENCE 



ical reaction. The burning of wood is another illustration of 

 chemical reaction. The formula for cellulose of which the 

 wood is composed is (C 6 Hio0 5 ). In burning, the oxygen 

 unites with the carbon to form carbon dioxide (C02), leaving 

 the other elements to form water. When iron rusts some of the 

 oxygen in the air unites with the atoms of iron, and actually 

 makes it heavier. 



35. Heat and Chemical Change. Heat is concerned in all 

 chemical reactions and either appears or disappears when the 

 reaction takes place. At very low temperatures, all chemical 



FIG. 4. Setting fire to zinc and sulphur on piece of asbestos paper. 



action ceases. In some cases, heat is necessary to make the 

 elements unite; in other cases, heat applied to a substance will 

 cause it to break up into its elements again; and in still other 

 cases, the union of the chemical elements gives off heat. The 

 union of hydrogen and carbon produces more heat than the 

 union of any other elements. The union of carbon and oxygen, 

 as in the burning of wood and coal also gives off much heat. 

 Sulphur and oxygen uniting give off much less heat and phos- 

 phorus and oxygen still less. Respiration is a familiar example 

 of the appearance of heat with chemical change, the heat being 

 due to the union of oxygen with the carbon in our bodies. Cal- 

 cium carbide, used in making acetylene gas, takes in much heat 

 energy when it is formed, but gives it off again when the gas 

 produced from it is burned. The rare element, radium, has 



