Movements of Molecules at very low pressures. 1047 



the y-activities of the radium with which it is being com- 

 pared. But, at 3*3 mm. of lead, thorium C of y-activitv = 

 1 mg. of radium and its products emits 3*07 x 10 7 a-particles 

 per second. Consequently, when comparison is made 

 through a greater thickness of lead, the rate of emission of 

 a-particles by thorium C is obtained by multiplying the 

 corresponding ordinate of curve VI. by 3*07 x 10 7 . The 

 curve obtained in this way is given in fig. 4, from which 

 maybe read the number of particles of range 8'6 cm. emitted 

 by thorium of y-activity = 1 mg. of radium and its pro- 

 ducts for comparison through any thickness of lead between 

 3'3 mm. and 14*3 mm. 



Our thanks are due to Mr. G. A. R. Crowe for the pre- 

 paration of the sources used ; and to Sir Ernest Rutherford 

 and Dr. J. Chadwick, whose valuable advice rapidly 

 eliminated our initial inexperience. 



Cavendish Laboratory, 

 Cambridge, 1921. 



CXII. Restricted Movements of Molecules at very low pressures : 

 A Limit of Applicability of the Second Laic of Thermo- 

 dynamics. By Arthur Fairbourne, Lecturer in Chemistry, 

 Kincfs College, London *. 



IT has long been realised and has often been stated 

 (compare Maxwell, 187 J, 'Theory of Heat,' p. 308) 

 that if sufficiently minute apparatus could be constructed to 

 restrict, selectively, the movements of certain individual 

 gaseous molecules so as to direct, for example, those with 

 high velocities into one particular portion of a system and 

 those with low velocities into another portion, then there 

 would result in consequence of this selection a continuous 

 creation of potential of temperature between these two 

 portions of the system ; a continuous creation of potential 

 which would render the kinetic energy of the molecules 

 continuously available for the performance of external work. 

 Purely hypothetical apparatus for the satisfactory theoretical 

 consideration of this possibility has been described, and 

 much discussed (compare Maxwell, loc. cit. ; Johnstone, 

 1921. ' The Mechanism of Life,' p. 215 ; Jeans, 1921, 

 ' The Dynamical Theory of Gases,' p. 183), since the 



* Communicated bv the Author. 



