27«i Mr. Norman Campbell on Delta Bays, 



with pressure depends on two factors, (1) the actual alte- 

 ration in the spatial distribution of the molecules, and (2) the 

 change in the degree of polymerization. If this second factor 

 is not negligible compared to the first, it would no. longer be 



HP 

 justifiable to consider the expression — —~ as referring to 



that part of the latent heat of vaporization which takes 

 account only of the transfer of the simple molecules from the 

 liquid to the vapour state ; and hence the conclusion that 

 the heat required for the depolymerizing process is approxi- 

 mately one half of the total, would no longer be valid. 

 Evidence — of a qualitative nature — in favour of the former 

 conclusion, however, is furnished by the *act that the change 

 of compressibility with pressure, especially at low pressures, 

 gives much the same iype of curve for normal and abnormal 

 liquids alike. At the same time the quite anomalous results 

 obtained with water have so far prevented any satisfactory 

 generalization being made. 



The results of the foregoing considerations may perhaps 

 be summed up as follows. 



The latent, heat of vaporization L of a liquid per gram is 

 connected with the compressibility /3 and the coefficient of 

 xpansion a x of the liquid by means of the relation 



ex r . 



L=- 



T «i 



The applicability of this relation appears, however, to be 

 restricted to normal liquids alone. 



XXIX. Delta Rays. By Norman Campbell*. 



1-3. Nature of the problem. 

 4-7. Experimental arrangements. 

 8-10. Incidence and emergence radiations. 

 11, 12. Influence of the speed of the primary rays. 



L Q -, Influence of the emitting substance. 

 14-16. The effect of reflexion. 

 17, 18. The real delta rays. 

 Summary. 

 Appendix on high resistances for electrostatic measurements. 



1. TT is now generally recognized that the slow moving 

 JL electrons emitted by a plate covered with a substance 

 emitting a rays, called by J. J. Thomson delta rays, represent, 

 at least in part, a secondary radiation excited by the alpha rays 

 in any solid body on which they fall. Numerous experiments 



* Communicated by the Author. 



