﻿Breaking Stress of Crystals of Rock-Salt. 65 



For rock-salt 



the density = 2*15, 



M= 1*64 x 58-5 xlO- 28 , 



p= ~ =2-24 xlO 22 ., 



Assuming that k is approximately = 0*7, the tension is 

 e 1 x 0*7 x 2-24 x 10 22 = 22'1 x 10" 20 X 0-7 x 224 x 10 22 

 = 3500 dynes per cm. (approximately). 



The figure obtained is of the same .order of magnitude as 

 figures derived by Ostwald and Huletfc (loc. cit.) for barium 

 salts. 



The normal pressure P would be accordingly 

 P = 98"7 X 10 9 dynes per square cm. 



It is interesting to see now how far the above results 

 agree with the experimental evidence. 



An experiment was performed as follows : — 



I took a good specimen of rock-salt crystal and I cut a 

 prism of the section about 15 square mm. and about 2-3 cm. 

 long. I used a suitable cement to hold the piece from both 

 ends, and by applying a suitable weight produced a rupture 

 of the crystal into two halves. Measuring the cross- 

 section of the rupture accurately, I calculated the weight 

 required to produce the rupture per square cm. I have 

 repeated the experiment many times with different samples 

 of rock-salt. If the crystal is well formed, the agreement 

 between individual experiments is fairly good. In one 

 series of experiments, I cut the prisms so as to have several 

 samples cut parallel to the three principal axes. I have done 

 the experiment with three pieces for each direction. For 

 one direction I obtained : 



89 lb. per cm. 

 103 „ „ 

 83 „ „ 



In the other two directions the results were identical. 



In some cases it happened that the rupture took place 

 under a much smaller weight. This, however, could be 

 attributed either to some faults in the structure of the 

 crystal, or to some other disturbances. Such measurements 

 were simply disregarded. 



Taking as the average value 91*7 lb., or 41*5 kgrs. per 

 square cm., one can calculate the inward pull per row of unit 

 length. 



Phil. Mag. 8. 6. Vol. 44. No. 259. July 1922. F 



