MATTER 9 



molecule of the compound is indicated by a figure written as a 

 subscript. 



14. States of Matter. — All molecules, and also smaller particles, are 

 believed to be in continual motion, but this motion is restrained by the 

 attraction which molecules or other particles exert upon each other. 

 This attraction is proportional to the sizes of the particles and inversely 

 proportional to the squares of the distances between them. Thus it 

 follows that particles at a great distance exert an attraction which is 

 practically negligible, but as they approach each other the attraction 

 increases at a constantly accelerated rate. 



The relation between the molecules in a mass determines the character 

 of the mass which they form, and thus we get the different states of 

 matter. If the molecules are sufficiently close together that the attrac- 

 tion between them holds them in the same relative position with respect 

 to each other, the mass preserves constantly the same form and is termed 

 a solid. If, however, the attraction is insufficient to preserve this form 

 and the mass tends to change its shape, it is called a fluid. If the mole- 

 cules of a fluid tend to remain together but there is so nearly a balance 

 between the force of motion and the force of attraction that the mass 

 easily changes shape, it is called a liquid. Under the influence of gravity 

 the molecules in a liquid seek the lowest level and the upper surface of 

 the mass becomes a plane surface. Finally, if the molecules of a fluid 

 are so far apart that the attraction of one for another fails to keep them 

 together and they tend to move in all directions, the mass expands and 

 fills all available space and is termed a gas. Thus both liquids and gases 

 are included under the term fluid. Some gases are heavy and expand, or 

 diffuse, slowly; others are light and diffuse rapidly. 



Since there may be any degree of attraction, depending upon the 

 sizes of molecules and the distances between them, these states of matter 

 are not sharply defined but pass into one another through an indefinite 

 number of gradations. A mass which is not a perfect solid but which 

 may be made to change its shape gradually is termed viscous. Under 

 the influence of varying degrees of heat and pressure, substances may be 

 made to assume any desired degree of viscosity or to pass from one state 

 to another. Tar may be heated until it flows readily, and a syrup cooled 

 until it hardly flows at all. Under the influence of heat, liquids may be 

 made to change to gases; and under the influence of increased pressure, 

 gases may be liquefied. Under ordinary conditions, some liquids readily 

 evaporate and change into gases, while some solids seem to pass directly 

 into the gaseous state without appearing at any time as liquids. In a 

 dry climate, snow may evaporate without wetting the surface on which 

 it lies. It might be argued that a substance must pass through the 

 liquid state in passing from the solid to the gaseous but there is difficulty 

 in proving this. 



