82 Mr. W. Ellis Williams on the Rupture of 
(3) Tresca. — Rupture occurs when the difference be- 
tween the greatest and least principal stresses 
exceeds a certain limit. 
(4) Coulomb. — Rupture occurs when the greatest shear 
has reached a certain value. 
The theory which has been most widely adopted is that 
attributed to Poncelet, which agrees closely with experi- 
mental results for a few substances, notably for cast iron. 
In this theor}' the extension is to be calculated from the 
applied stresses by means of the ordinary elastic constants, 
and the theory requires for each body only one other constant, 
the limiting extension. 
Some years ago Voigt * made a series of experiments on 
the effect of hydrostatic pressure on the breaking strength or 
rocksalt and of wax, and obtained results in direct opposition 
to Poncelet's theory. Voigt measured the breaking-stress 
of rocksalt specimens enclosed in a cylinder containing 
carbonic acid gas under a pressure of 50 atmospheres, and 
found that the tension necessary to break the specimen was 
the same as when the experiment was performed under 
atmospheric pressure, while according to Poncelet's theory 
it should be twice as great. Voigt concludes from his 
experiments that the third theory due to Tresca is the most 
satisfactory. The results could, however, be equally well 
explained by Coulomb's theory, as both give nearly the same 
result for this kind of stress combination. The following 
experiments were carried out to confirm and extend Voigt' s 
results, and especially to repeat them under the highest 
possible hydrostatic pressure. The range of pressure obtained 
in the present experiments is 900 atmospheres or 9 kg. per 
sq. mm., which is twenty times the breaking-stress of rock- 
salt, the material experimented upon. 
Description of Apparatus. 
The hydrostatic pressure was obtained by means of a 
Schaffner and Budenberg screw-pump furnished with a 
Bourdon gauge, reading to 1500 atms. 
The specimens were tested in a steel cylinder of the 
following dimensions : — Inner diameter 4 cms., outer diameter 
10 cms., length 80 cms. 
The cylinder was fixed vertically in an iron stand built 
into the stone floor of the room, so as to avoid all shocks and 
vibrations which might tend to break the test-pieces. The 
specimens were ruptured by means of the electromagnetic 
* Voigt, Annalen der Physik, lvii. p. 452 (1899). 
