9. JUNIOR GRADE SCIENCE 



Solids. Things which have a size and shape of their own, and 

 remain of the same size and shape so long as they are not interfered 

 with, are called solids. Some solids are harder than others, and some 

 can have their shape altered more easily than others. But none of 

 them change by themselves. In fact, a solid body does not readily alter 

 its size or shape. It keeps its own volume and the same form unless 

 acted upon by a considerable force. 



Another way of expressing the chief facts contained in the definition 

 is to say that solids possess rigidity. Because of its rigidity every solid 

 has a definite free surface and a definite shape. 



Liquids. If a wine-glassful of water is poured into a tumbler the 

 water fills the lower part of the tumbler. Originally the water had 

 the shape of the wine-glass, but after the transfer it takes the shape 

 of the lower part of the tumbler. And, if it be poured successively 

 into vessels of different shapes, the water always assumes the shape of 

 the lower part of the containing vessel. Throughout all the changes 

 of shape there is no alteration of size ; for if at the end of the experiment 

 the water is poured back into the wine-glass it exactly fills it. 



The surface of a liquid at rest is always horizontal if the vessel 

 containing it is not very small. This is one of the results of the 

 fluidity, or power of flowing, possessed by liquids in common with 

 gases. When not held by the sides of the containing vessel a liquid 

 at once flows. 



These facts provide a definition which includes all the important 

 characters of liquids. A liquid at rest has a horizontal surface and takes 

 the shape of the part of the vessel below the surface, but the conditions 

 remaining the same, the liquid keeps its own size or volume, however 

 much its shape may vary. 



Not only is the power of flowing possessed by fluids, but also by 

 some powders. Fine sand may be made to flow, but the particles 

 do not move freely over one another, so the surface is left uneven. 

 Indeed the horizontal surface of liquids at rest provides a clear means 

 of distinguishing between a solid and a liquid. 



Gases. A liquid always adapts itself to the shape of the containing 

 vessel, and presents a level surface at the top ; a gas, on the other hand, 

 will, however small its volume, immediately spread out and do its 

 best to fill the vessel, however large ; and it does not present any 

 surface to the surrounding air. We can never say exactly where the 

 gas leaves off and the air begins. 



Gases possess fluidity to a much more marked degree than 'liquids. 

 Liquids and gases are, as has been seen, both classified as fluids. 

 But, whereas liquids are almost incompressible, gases are very easily 



