i8 MOLECULAR WEIGHT AND POLYMERISM 



sixth of the molecules contained in a parallelepiped at 

 right angles to the surface, and C metres long ; i. e. 



NC 



67* 



They are repelled with equal velocity; so that the whole 

 have in the second suffered a change of velocity of 2(7 

 metres per second. The force needed for that (P kilo- 

 grams pressure per sq. metre) may, according to the law 



force = mass x acceleration, 

 be calculated as 



P- , j- -~ .-u -r\TT 



= -^MxzC = T7 - or PV= 

 OK 3 V 



where M is the mass of a molecule. 



For two gases at the same temperature and pressure we 

 have consequently : 



The third condition, equality of temperature, includes, 

 however, a further relation, for it may be shown that 

 equality of temperature corresponds to equality of mean 

 kinetic energy of the molecules, or 



(2) \MW = \M*, 



from which by combination with (i) we get 



^ = N 2> 



that is an equal number of molecules for equal volume, 

 pressure, and temperature. 



Before we describe the methods in which Avogadro's law 

 may be applied, we may conveniently put it in a single 

 mathematical form which expresses also the laws of Boyle 

 and Gay-Lussac. It is well known that the last two may 

 be represented by 



If the molecular quantity of different gases be taken, then 

 according to Avogadro's law, as just stated, for equal 

 pressure (P) and temperature (T), the volumes (V) will 



