THE MEANING OF PERCEPTION 167 



the acceleration of a body possessing mass; what we observe is 

 that something that is at rest begins to move, or that something 

 that is moving already moves more quickly or more slowly, and 

 that is what we mean by saying that some force acts on the body. 

 Mass itself is only measured by observing that something would 

 move towards the earth if it were free to do so, and then by 

 preventing it from moving by balancing it against something 

 else — that is, by weighing the body having mass. Flow of heat 

 is only the transference of rapid motions of the molecules of one 

 body to the molecules of another body which are moving less 

 rapidly. Chemical reactions are displacements of the atoms 

 within the molecules of substances; thus, when we add some 

 solution of silver nitrate to solution of common salt a white 

 precipitate is formed thus : 



AgN03-}:NaCl=AgCl+NaN03, 



the atoms of silver and sodium being mutually displaced with 

 relation to the NO3 and CI. Or chemical reactions may be the 

 coming together of atoms to form molecules, as when the gaseous 

 atoms of hydrogen and iodine combine to form hydriodic acid gas : 



H2+l2=2HI. 



Or they may be the breaking up of molecules into atoms, as 

 when the last reaction is made to reverse itself : 



2HI=l2+H2; 



or into other molecules, as when the salt, ammonium chloride, 



dissociates : 



NH4C1=NH3+HC1, 



and so on in a very great number of ways. All chemical changes 

 include displacements or rearrangements of atoms, and such 

 changes are accompanied by various physical effects. Heat is 

 generally evolved, or is absorbed, when the rearrangement occurs, 

 and that means that the rapidity with which the molecules of 

 the reacting substances were moving changes. Or the latter 

 may change their state ; thus, when petrol is set on fire its mole- 

 cules, which were previously in the liquid state, react with the 

 oxygen of the atmosphere to form molecules of water and 

 carbonic acid gas, the atoms being rearranged. At the same 

 time the new molecules become gaseous — that is, their freedom 

 and rapidity of motion increase. Or sound may be produced, 

 as when dynamite explodes, and that means that a large quantity 



