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use of the word leii;i^lli of path. If we a\<>i<l tin's, every one will 

 see throiifj^h tlie niistuke; for i-eiilly the reason inii' e(nnes to this: In 

 a certain time a numl)er of [)artieles j*eaeii a eerlain snrface .l;now 

 every partiele reaches a snrface /> somewhat sooner, so more particles 

 I'each ]j than A in the same time. In essentially the same way we 

 iniu'hl show that more vertical fallin<2; raindrops would strike a pointed 

 roof than a flat one of a section of equal area. Nobody will make 

 this mistake, because it is cleai- that the number of raindrops fallinji,' 

 ()]! the roof depends solely on the (piaiitities, Avhich <;overii the 

 stream of those drops and on the area of the section of the roof. 

 The same holds foi" the niolecules ; and tluMpiantities which determine 

 the strength of the stream of molecules are only the velocity of 

 the molecules, the law^ of the distribution of velocity which is the 

 same, whatever the form be of the molecules, and the number of 

 molecules per unit of volume. 



I said already, that the opposed opinion derives its force from the 

 ambiguous use of the Avord "length of path." We might also justly 

 say of the raindrops that the path which they describe to get from 

 a certain point to the roof, is shortened, wdien w^e think the roof 

 pointed. In the same way we may say of the molecules, that the 

 path from a certain Ji,red point to the sphere is shorter than that to 

 the disc, but this does not hold for the mean path and of course 

 the latter only is iji inverse ratio to the numl)er of collisions. The 

 A'alidity of this reasoning is easy to see by means of any of the 

 seemingly different definitions for the length of path. I shall for 

 shortness, confine myself to that according to which the mean length 

 of path of molecules moving with the velocity c, if? found by 

 examining, how^ many molecules strike against a certain molecide 

 within a certain interval of time, by adding the })alhs described by 

 any of those molecules between this collision and the preceding one, 

 and by dividing this sum by the number of those molecules. Let us 

 now^ determine the mean path of the molecules which strike with a 

 Aelocity c from a direction P 1^ either against the disc S, or the 

 spherical surface B (Fig. 1). We shall call these molecules the 

 molecules of the group in question. The fact that a molecule of the 

 group in question whose last collision before it reached 7>or>S'took 

 place in a point ^4 ^), will have a shorter i)ath to B than to S, does 

 not call for discussion. In so far van dek Waals and Korteweg are 



^) We shall call such a point henceforth the "last point of collision". This is 

 therefore the point where the collision takes place, which makes a molecule pass 

 into the "group in question". The collision with B or S makes the molecule leave 

 this group again. 



