Materials under Combined Stress. 85 
line, and consequently the axes of the four steel points also 
lay in this line. 
The whole was then placed on a watchmaker's pair of turns, 
and by means of a pulley on one of the pieces the rocksalt 
was turned with a file and then polished with a piece of moist 
wood. The pieces were turned down a little thinner in the 
middle as shown in the figure, the diameter of the thinnest 
part being about 2 or 3 mms. This process of preparing the 
rocksalt ensured that the two points from which it was sus- 
pended were exactly in the same straight line with the axis 
of the rod. 
As the fluid used in the pump and cylinder was water, it 
was necessary to protect the rocksalt from its action. For 
this purpose a small brass tube was screwed on the lower of 
the two aluminium pieces, and the vessel thus formed was 
filled with nitrobenzol, which completely covered the rock- 
salt. It was previously ascertained that this fluid had no 
dissolving power for rocksalt and did not affect its breaking 
strength. 
Results. 
It was found from the beginning that the value of the 
breaking tension varied very much for different specimens. 
At first it was thought that this variation was due to imper- 
fections in the apparatus and methods of preparation of the 
rocksalt rods. All improvements in the apparatus did not, 
however, alter the results, and the conclusion was finally 
come to that the differences were due to variations occurring 
in the rocksalt itself. 
Two different crystals were used in the experiments — one 
about 6 x±x'2 cms., and the other a large square block about 
10 cms. each way. 
The first piece was all used up in preliminary experiments 
in determining the breaking stress of rocksalt with weights, 
and the results obtained were much better than the results 
afterwards obtained in exactly the same way with rods cut 
from the larger block. It has been shown by Voigt that the 
breaking stress of crystal specimens depends on the direction 
in which they are cut relative to the crystalline axes, and 
that in rocksalt this variation is very marked. I found that 
in specimens from the large block cut in a direction making 
an angle of 45° with the crystalline axes, the breaking stress 
was 4000 grammes per sq. mm., or more than four times the 
maximum result with pieces cut parallel to the axes. It is 
possible therefore that the variations observed are due to 
small irregularities in the crystalline structure of the pieces 
