4 A YEAR IN SCIENCE 



rately weighed. With an air pump the air is exhausted 

 from the globe and the stop cock is closed. The globe 

 is again weighed. It is found to weigh more when filled 

 with air than it does after the air has been exhausted. 

 Air therefore has weight. 



Since air occupies space and also has weight, according 

 to the definition of matter, air is matter. Air is a gas, so 

 we have proved that a gas is matter. 



Three forms of matter. All material exists in one 

 of three forms : solid, liquid, and gas. If a substance has 

 definite size and shape, we call it a solid. The shape of a 

 solid can not be changed except by some external force. 

 If the size of a substance is definite, but the shape depends 

 upon the containing vessel, it is a liquid. Gases have 

 neither definite size nor shape, and they expand without 

 limit. 



The behavior of these three forms of matter, and the 

 changes which they undergo under different conditions, 

 will be studied in the following chapters. 



Questions 



1. With what does the subject of science deal? 



2. Into what two divisions is science naturally 

 divided ? 



3. What is meant by the words animate and 

 inanimate ? 



4. Name the two natural sciences. How do they 

 differ from each other? 



5. Name three physical sciences. Of what does 

 each treat? 



