I PHYSICAL PROPERTIES AND STATES OF MATTER 15 



glass, and then place one on the other. The two surfaces will 

 be found to cohere, and considerable force will be required to 

 separate them. 



DISTINCTIVE CHARACTERS OF GASES 



Characters of Gases. It has already been explained that the 

 leading difference between a solid and a liquid is the power of 

 flowing which the latter possesses. Gases also possess fluidity, 

 and to a much more marked degree than liquids. But whereas 

 liquids are almost incompressible, gases are very easily com- 

 pressed into a much smaller space according to a definite 

 law, viz., just in that proportion in which you increase the 

 pressure on a gas do you decrease the volume which it occupies. 

 Nor are these the only differences. 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 a vessel, 

 however large ; and it does not present any surface to the sur- 

 rounding air. We can never say exactly where the gas leaves off 

 and the air begins. Another distinction will be more fully 

 appreciated after we have considered the action of heat upon 

 the volume of bodies. We shall learn that, generally speaking, 

 all bodies get larger as they are heated ; this is very much more 

 decidedly the case with gases than with liquids. Gases, then, 

 are easily compressible and expand indefinitely. 



We shall learn that gases expand equally when heated to the 

 same extent, but this fact and others will be much better under- 

 stood in their proper places. 



Constancy of Weight in different States. When a solid is 

 converted into a liquid, or a liquid into a vapour, no change of 

 weight is experienced. This has been found to be true in all 

 cases, and the following experiments will illustrate the fact : 



EXPT. 20. Boil water in a flask or a retort, as in Fig. 10, 

 and catch the condensed steam, taking care that none escapes, 

 in another flask kept cool by resting in water. The water thus 

 collected will be found to have the same weight as that boiled 

 away. 



EXPT. 21. Place a piece of ice in a flask suspended from 

 one arm of a balance. Counterpoise the flask with the ice in 



