MATTER AND FORCE 



153 



clearness and conciseness of expression 1 . This is necessary 

 in formulating definitions of the properties of matter such 

 as these: 



Density of matter refers to the compactness of its molecules, or the amount 

 of matter in a given space. 



Size, or volume, refers to the amount of space occupied by a body. 



Impenetrability is the property that does not permit two bodies to occupy 

 the same space at the same time. 



FIG. 52. Masses the same as shown by the position at O of the movable 

 arm P t but densities different. 



Extension is the property manifest in the fact that a body does occupy 

 space. 



Form results from a limitation of the extension of a body in different direc- 

 tions. If matter extended outward indefinitely it would be without form. 

 There would then be no separated portions of matter, or bodies. 



Mobility refers to the fact that bodies are movable, i.e., they can be made to 

 change positions. 



1 From simple experiments performed in Physics it is possible to acquire 

 groups of facts which call for discrimination in the use of words to express what 

 has been learned. Loose use of language, and the incomplete indefinite grasp 

 of facts usually accompanying any such use, defeat one of the most important 

 purposes in the study of high school sciences. On the other hand, an ability to 

 repeat the carefully chosen words of a text-book in science is no proof of a full 

 understanding of their meaning. Wrong notions, or imperfect ones, obtained 

 from a misinterpretation of the readings of a text are not easily corrected when 

 once acquired. Special effort should be made in all studies in elementary 

 science for a right choice and use of terms, and for a definiteness in knowledge 

 concerning what is expressed in these words. One of the valuable results 

 of wisely chosen laboratory exercises is the acquisition of a vocabulary of exact 

 scientific terms expressive of what has been learned through experimental 

 effort. 



