86 Mr. W. Ellis Williams on the Rupture of 
which cause the direction of the axes to vary from point to 
point of the crystal. Although owing to these variations it ! 
was impossible to make any exact comparison between the 
breaking strength with and without external pressure, yet 
as the hydrostatic pressure employed was many times greater 
than the breaking stress of rocksalt, the results show clearly 
that the phenomena are not in accordance with Poncelet's 
theory. Thus the strength of rocksalt under atmospheric 
pressure varied from 400 to 900 grammes per sq. mm. with 
a mean of 600 grammes. According to Poncelet's theory 
the strength under 1000 atmospheres should be 10,000 
grammes, while the values actually obtained range between 
500 and 1000 grammes. 
The actual results obtained are shown in the following 
table : — 
Tests without external pressure '. 
Specimens from first block, broken with weights : — 
449, 372, 390, 391, 497, 466, 530, 510, 407, 364, 
485, grms./sq. mm. 
Mean : — 447 grms./sq. mm. 
Specimens from second block of salt, broken with weights : — - 
503, 600, 901, 940, 570, 381, grms./sq. mm. 
Mean : — 649 grms./sq. mm, 
Specimens from second block, broken in the cylinder with 
electromagnetic apparatus, but without pressure : — 
630, 700, 700, 400, 720, 380, 300, 427, grms./sq. mm. 
Mean : — 537 grms./sq. mm. 
Tests under pressure. 
All specimens from second block. 
Pressure. Breaking Stress. 
150 atms. 932 grms./sq. mm. 
200 
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502 
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500 
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501 
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960 
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800 
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965 
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840 
970 
J5 
540 
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900 
?J 
587 
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980 
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