204 Messrs. A. F. Joffe and M. V. Kirpitcheva on 



either that the free path of the electrons is at least a hundred 

 times as large as that of the molecules, or that a larger pro- 

 portion of the electrons do not lose any appreciable part of 

 their energy upon collision. 



In view of the above, a question arises as to the gradual 

 hardening of an X-ray bulb with usage. It is usually 

 assumed that the cause of this is the disappearance of the 

 gas in the bulb, but this assumption is not based on direct 

 experiments. It seems more probable that the gradual 

 hardening is due to the phenomenon described in the present 

 paper. 



My thanks are due to Prof. W. L. Bragg for the interest 

 he has taken in this work. 



The Physical Laboratory, 

 Victoria University, 

 Manchester. 



July 1921. 



XVIII. Rontgeno grams of Strained Crystals. By A. F. 

 Joffe, Member of the Russian Academy of Sciences, Di- 

 rector of the Rontg etiological and Radiological Institute, 

 Petrograd, and M. V. Kikpitcheva *. 



[Plate II.] 



I. The Residual Strain. 



IT is a well-known fact that several crystals are capable of 

 undergoing a residual strain without any apparent loss 

 of their integrity. Thus, for instance, rock-salt becomes 

 plastic like wax on being warmed or under the action of a 

 slowly increasing load. In spite of numerous investigations 

 on the nature of these strains by crystallographers, the 

 question how the structure of the crystal can remain 

 deformed after strain, when no external forces exist, remains 

 unanswered. An investigation of the crystal between 

 crossed nicols shows that great internal stresses, which 

 might explain the strain, do not exist in it. The suppo- 

 sition that a sliding of separate layers of the crystal 

 along the plane of the rhomboic dodecahedron takes place 

 in such a manner- that the regularity of the crystalline 

 grating remains in spite of the external change of form, is 

 shown not to hold by the fact that on bending a crystal of 



* Communicated by Prof. Sir E. Rutherford, F.R.S. 



