Rontgenograms of Strained Crystals. 205 



rock-salt the crystal breaks not along planes, but along 

 curved surfaces, which do not straighten under a prolonged 

 annealing. 



To solve this question we have used Laue's rontgeno- 

 grams, observing the picture on a fluorescing screen or on 

 a photograph. It is most simple to study a homogeneous 

 strain [e.g. a compression) of rock-salt, loading it gradually 

 by means o£ an electromagnet. 



After the limit of elasticity is passed, the separate spots of 

 Laue's rontgenograms begin to elongate and to form the 

 figure shown on the accompanying photograph (PL II.). 

 The detailed photograph of a crystal not too much strained 

 shows in every elongated spot a stratification, consisting of a 

 series of separate spots the number of which increases with the 

 increase of the strain. Every such beam is at the same time 

 a spectrum (which can clearly be seen on the photograph), 

 but it is easy to observe that the stratification is due to 

 separate small crystals, into which the whole crystal has 

 parted, and is not a line spectrum of Rontgen rays. The 

 dimensions of the fragments are a few hundredths of a 

 millimetre. 



On the fluorescing screen and on the photograph the 

 direction in which the small crystals slide can be seen. The 

 spot corresponding to the rhombical dodecahedron (110) 

 remains completely unaltered, while the others become elon- 

 gated. The direction of the stratification in every beam 

 also shows the direction around which the rotation of the 

 little crystals takes place. Thus we come to the conclusion 

 that a crystal of rock-salt when compressed breaks up into 

 small crystals, which, while sliding along the plane of the 

 rhomboic dodecahedron, rotate at the same time through 

 different angles. 



We have also shown that the structure of these small 

 crystals is the same as that of the original crystal. For this 

 purpose an Ag line Rontgen spectrum was photographed 

 from the plane (110) both of an unstrained and a strained 

 crystal. Both spectra coincided : thus the constant of the 

 crystalline grating did not alter. Similar results were 

 obtained for calcium sulphate and also for a series of natural 

 minerals with irregular faces. Quartz does not show any 

 diffused spots until it breaks, its limits of elasticity and 

 durability being the same. 



The method of direct observation is a very convenient one 

 for the determination of the limit of elasticity of different 

 materials and also for the study of the manner of their 

 destruction. Observations have shown, e.g., that the 



