176 REPORTS ON THE STATE OF SCIENCE.—1913. 
Adams,® and Nicholson,®° and Coker ® tested rocks in compression, and 
supplied lateral support by enclosing each specimen in a steel cylinder 
which bulged laterally. Marble flows as a plastic body under differential 
pressure by distortion of the calcite grains, and the deformed specimen 
retains 60 to 85 per cent. of its original compressive strength. Its specific 
gravity is not increased. By Kick’s process—in which the specimen is 
embedded in a fused salt, usually alum, to fit the retaming cylinder— 
minerals with hardness under 5 show plastic deformation, which is less 
pronounced as they are harder. Still harder minerals, which do not flow, 
have their structure broken down and are powdered. Fine-grained, 
massive limestones show combined flow and fracture. Harder rocks, like 
granite, crumble under pressure, but the flow structure is developed in 
these by greater differential pressures. 
10. The Friction Theory. 
Many investigators have studied the internal friction of solids, and 
when not associated with combined stresses the favourite method has been 
by the decay of torsional oscillations. Only a few references are given 
to the large volume of research of this type. Lord Kelvin poimted out 
that the damping was caused by all the effects included under the class of 
hysteresis phenomena. Bouasse dealt with torsional oscillation, and paper 
No. 32 includes a review of his work. Ercolini*® again pointed out that 
the damping is due to hysteresis, and not to molecular friction. Guye’s 
work is of a similar character. 
Reference has been made to the angles of fracture of brittle materials in 
compression, which probably suggested Theory (d), and to the equality 
of the yield stresses for steel in tension and compression. 
In connection with combined stresses, Scoble 4° © considered that the 
friction theory does not apply to steel. Gulliver*’ found that steel yields 
in tension at an angle of 50 degrees to the axis (u = 0-176), but this is not 
confirmed by yield at 40 degrees in compression. A study of combined 
stress experiments led him to the same conclusion as that of Scoble, since 
calculated values of ‘ 1’ varied from —0-242 to0:38. Smith’s tests did 
not support the friction theory for steel, nor did those of Mason ** which 
were specially well adapted to test it. 
11. Liider’s or Harimann’s Lines. 
These markings have been studied in this country chiefly by 
Gulliver ** ®7 in relation to the friction theory, and by Mason” in connec- 
tion with his combined stress experiments. Their papers will furnish 
further references. 
12. Some other Considerations in Combined Stress Researches. 
The peculiarities in the behaviour of steel—variation of the elastic limit, 
hysteresis, &c.—have been discussed elsewhere. It is possible that their 
importance has been magnified, since the elastic limit and yield point 
coincide approximately for thoroughly annealed steel, and the hysteresis 
effect is extremely small. The difficulties are intensified in the case of 
-other metals, because most have no elastic range and no well-defined yield. 
Apparently we must study the fracture of these materials under uniform 
stress distribution. Brittle substances, like rocks, cement, &c., ure 
