60 Introduction to the Study of Science 



in such a way as to yield a new product. The burning wood, 

 for example, yields light, heat, smoke, and ash, all of which 

 are new and different from the wood and the air which combines 

 with it in burning. Such changes are different, moreover, 

 from the merely physical changes such as pouring sand from 

 one bucket into another, sawing wood, or whipping eggs to a 

 stiff froth (page 131). 



There are still other ways of producing heat. You know 

 that if you rub your hands together briskly, they become 

 warm, and that by rubbing a piece of metal on cloth or wood, 

 heat is produced. By hammering a piece of metal such as lead 

 or iron it is made warm, and may be made too hot to hold by 

 the continued impact of the blows. When a tire is pumped very 

 tight, the air is compressed and yields heat. Thus by friction, 

 impact, and compression, heat is produced. 



A very small amount of heat is derived from the interior of 

 the earth. The central mass of the earth, though solid and 

 not molten, is supposed by scientists to have a temperature 

 of possibly 36,000 F. Heat from this source may be released 

 through volcanoes, geysers, and hot springs as well as in other 

 less conspicuous ways. 



30. Kindling a fire. The commonplace procedure of kin- 

 dling a fire involves a more or less definite knowledge of an 

 important fact which is often described as the kindling tem- 

 perature of substances. This fact we understand in a general 

 way to mean that a given substance, such as paper, wood or 

 coal, begins to burn at a certain temperature. What this 

 means in particular may become clear in the following exercise. 



Exercise. What are the usual means of starting a fire ? How did 

 primitive people produce fire? Try to kindle a fire with a rapidly, 

 rotating stick on a board and some inflammable material. Can it be 

 done quickly ? Why were special efforts made by people of the ancient 

 world to keep a fire overnight or for longer periods? Try starting a 

 fire with a reading glass. Upon what sort of material must the sun's 

 rays fall to kindle readily? How does a reading glass differ from a 

 piece of ordinary window glass in respect to form and effects? 



