SECTIONAL TRANSACTIONS.— A. 387 



Three layers of lead in the chamber are spaced so that the successive stages 

 of the multiplication process can be seen. Of two thousand showers photo- 

 graphed, three were of a markedly different type, diverging from a point 

 at more or less random angles and containing heavy particles ; these cannot 

 be explained by the multiplication hypothesis. 



It has been known since the experiments of Bothe and Kolhorster and 

 of Rossi that cosmic rays contain single corpuscles of much greater pene- 

 trating power than the radiation theory allows for electrons. More detailed 

 information about the penetrating power has been obtained by Street and 

 Stevenson. Two cloud chambers are used ; in one the momentum is 

 measured by the curvature in a magnetic field, and in the second the pene- 

 tration through layers of lead is observed. A number of particles are found 

 which must be supposed to be neither electrons nor protons. Various 

 observers have obtained tracks from which the mass could be estimated by 

 the density of ionisation ; values obtained are about two hundred times the 

 electron's mass. 



Prof. W. BoTHE. — New results in cosmic rays (11.30). 



Dr. E. J. Williams. — The heavy electron (11.45). 



General Discussion (12.0). 

 Dr. R. W. Wood. — Crystal growth (film) (12.30). 

 Dr. K. T. Fischer. — The temperature coefficient of balances (12.45). 



Afternoon. 



Visit to the Observatory, Solar Physics Observatory, Pendulum House, 

 and Cavendish Field Laboratory. 



Monday, August 22. 



Discussion on Low-temperature physics, with special reference to Helium II 

 (lo.o). 



Dr. H. B. G. Casimir. — Introduction : Low temperature properties of 

 matter. 



The problems of low temperature physics can be divided into two groups : 

 those depending exclusively on the motion of atoms and molecules as a 

 whole and those connected with the inner degrees of freedom of the atom. 

 The study of electrons in metals and of paramagnetism are the two most 

 important examples of this second group. In the limit of very low tem- 

 peratures (T < 4° K) the properties of a non-conducting non-paramagnetic 

 solid are comparatively simple. The heat motion can be described as a 

 superposition of sound waves ; the specific heat is proportional to T^ and 

 can be calculated from the elastic constants. Also the theory of heat 

 conduction becomes very simple. 



It is to be expected that at this limit the interaction between the lattice 

 and the inner degrees of freedom will also be simplified. The theory of 

 electrons in metals leads to the result that the resistance due to interaction 

 with lattice vibrations decreases very rapidly (~T*). In the case of para- 

 magnetism, the lattice vibrations come into play only in so far as they must 



