110 MOLECULAR MOTION AND ITS ENERGY 48 



cohesion, we do not on this account need to drop the hypo- 

 thesis of rectilinear motion, in which the most essential and 

 characteristic peculiarity of Bernoulli's theory consists. 

 It only becomes necessary to modify the hypothesis so that 

 the changes of direction from one straight path to another 

 are not caused suddenly by a collision, but gradually by 

 forces which act continuously, even if they very quickly 

 come into and go out of play. The paths then do not form 

 sharp-angled zigzag lines, but the passage from one straight 

 line to another is brought about by lines that are sharply 

 but continuously curved so that the corners seem to be 

 rounded off. In the next paragraph we shall have to 

 discuss, at least in its general character, the influence which 

 this modification of the hypothesis introduces into the cal- 

 culation of the pressure. 



Still, an effect of quite a different kind is conceivable 

 with the same hypothesis as to the nature of cohesion. 1 If 

 we assume, with regard to this force, that it acts only in the 

 proportionately rare moments of an actual or very nearly 

 occurring collision between two molecules, the fact esta- 

 blished by Joule and Lord Kelvin, that the intensity of 

 the cohesion in gases is very small, will be simply explained 

 by the force acting only during the short time of the 

 collision and being in abeyance during the much longer 

 interval between successive collisions. There would there- 

 fore be no contradiction with the observations mentioned if 

 we assume that in the short periods of collision the force 

 acts with very considerable intensity. 



But if this assumption is admissible there is nothing 

 inconsistent in the hypothesis that the attractive forces of 

 cohesion might be able, at least now and then under favour- 

 able circumstances, to bind together two colliding particles 

 so fast that they traverse the next stretch of their path 

 together as a double molecule. By this a state of equi- 

 librium would be produced in the gas in which, among the 



1 In his memoir on ' Temperature and the Measure of Temperature ' (Pogg. 

 Ann. Erg.-Bd. vi. 1874, p. 275) Eecknagel also considers effects of two kinds 

 attraction of molecule on molecule and actions within molecules. The latter 

 are perhaps to be interpreted in the manner explained later. 



