Fire and Heat 131 



eighty known at present. Compound substances are almost 

 beyond enumeration. 



Physical and chemical change. All compounds are the 

 products of what scientists describe as a chemical change. 

 This is very different from a physical change. The candle 

 may be melted into a shapeless mass of paraffin, and then 

 restored to its original form with practically no loss of material. 

 When, however, the candle is burned in air, its structure is 

 forever destroyed, and by no art or trick of magician or scientist 

 can it be restored from the products. Melting the candle is 

 a physical change which gives nothing new ; burning is a chemi- 

 cal change which produces new compounds. 



The distinction between the two kinds of change may be 

 emphasized by another example. One may pour water from 

 one vessel into another, or carry water from the Atlantic 

 Ocean to the Pacific, but one does not change its structure or 

 produce a new compound. Water may be changed into a 

 solid, liquid, or gaseous state ; but its structure remains un- 

 changed. It is still water in any of these states. But when 

 water is decomposed by electricity or other means, it ceases 

 to exist as water ; and in its place are two gases, invisible and 

 odorless, one supporting combustion, the other highly com- 

 bustible. This is a chemical change. Again when hydrogen 

 is burned in oxygen, both are changed structurally or chemically, 

 and produce a new compound substance. Decomposition is a 

 chemical change. Production of new substances is a chemical 

 change. Change that does not affect the structure of a sub- 

 stance or produce new substances is physical. There are many 

 examples of both kinds of change in the world about you, for 

 example, cooking potatoes, baking bread, mixing soap and water, 

 sawing wood, or breaking stone. 



52. Carbon dioxid. The study of how fire burns and of 

 the products of burning brings to light, among other important 

 facts, that the mixture which we call air consists of nitrogen and 

 oxygen in large proportions, and carbon dioxid in much smaller 



