90 MOLECULAR MOTION AND ITS ENERGY 38 



The thermal effusion therefore ceases as soon as the ratio 

 of the pressures on the two sides of the partition has 

 attained the value of the ratio of the square roots of the 

 absolute temperatures. 



39. Heat Effects Accompanying Effusion 



As effusion may arise from inequalities of temperature, so 

 may it cause the temperatures at the two sides of a porous 

 partition to be unequal. Cooling is produced at the side 

 from which the flow takes place, while the other space into 

 which the gas streams is warmed. The same action there- 

 fore occurs which has already been described in 18, 19. 

 Where the gas expands it cools, and where it is condensed 

 it gets warmed. 



The explanation of this behaviour 1 is also essentially 

 that which has been given for the case in which it was 

 assumed that the condensation was caused by the pushing 

 down of a piston, the lifting of which made the gas to 

 expand. The only difference consists in our having to take 

 into account the encounters of the streaming particles with 

 each other and with other particles instead of the collisions 

 of the molecules against the piston. 



A particle which reaches .the orifice from the interior of 

 the receiver does not here meet particles at rest, but particles 

 in motion that are proceeding in the same direction. In 

 consequence of this the particle will be thrown back into 

 the receiver, not with the same speed with which it arrived 

 at the orifice, but with a much less speed, while the motion 

 of the particle that streams out is increased. Thus the 

 particles in the receiver lose part of their molecular energy 

 during the flow, and the gas in the receiver therefore cools. 



On the other side of the partition the particles that 

 escape from the orifice strike against particles that either 

 are at rest or have lost in the larger space at least a part of 

 their energy of flow by its transformation into heat. To 

 these latter particles momentum is communicated by those 

 which rush from the orifice with the full speed of the 



1 L. Natanson, Wied. Ann. xxxvii. 1889, p. 341. 



