Matter and Energy. 1 1 



vessel, in number and nature corresponding to the sides of 

 the vessel. The one free surface is always horizontal, and is 

 constantly changing owing to the escape of molecules from 

 the surface into the air. A gas has no free surface. The 

 molecules, freed from mutual influence, are constantly tend- 

 ing to become farther and farther separated from each other. 



It has been calculated that in a cubic inch of air the 

 number of molecules can be expressed only by a row of 

 figures twenty-one in number. This calculation can give 

 only a very faint conception of the number of molecules in 

 a given mass, or of their extreme minuteness. Each molecule 

 is believed to be in perpetual and extremely rapid motion in 

 straight lines in all directions, and to hit against its neigh- 

 bours on every side at the rate of something like 18,000 

 million times per second. 



In addition to this motion of translation, molecules are 

 believed to possess intrinsic motions of vibration and rota- 

 tion, dependent for their intensity upon the temperature of 

 the substance. 



As already stated, an application of heat to a solid has 

 the effect of transforming it into the liquid, and through 

 that state into the gas, or, in some cases, into a gas directly, 

 the molecules of which can by further application of the 

 same force be made to recede farther and farther from each 

 other. Change of state is therefore accompanied by change 

 in the rate of motion of the constituent molecules, a high 

 temperature being associated with a rapid motion, and a 

 low temperature with a relatively sluggish motion. The 

 converse is likewise true, abstraction of heat being sufficient 

 to change the gaseous state first into the liquid and lastly 

 into the solid. Water may be again cited as the most 

 familiar example of this change. 



The transformation of one state of matter into another is 

 possible in virtue of a manifestation of energy. 



