72 



Royal Society. 



Southward, I was much struck with the appearances presented by 

 some specimens of iron and steel round bars which had been broken 

 by torsion. Some of them were broken right across, as nearly as 

 may be in a plane perpendicular to the axis of the bar. On examin- 

 ing these I perceived that they had all yielded through a great de- 

 gree to distortion before having broken. I therefore looked for bars 

 of hardened steel which had been tested similarly, and found many 

 beautiful specimens in Mr. Kirkaldy's museum. These, without 

 exception, showed complicated surfaces of fracture, which were such 

 as to demonstrate, as part of the whole effect in each case, a spiral 

 fissure round the circumference of the cylinder at an angle of about 

 45° to the length. This is just what is to be expected when we 

 consider that if A B D C (fig. J ) represent an infinitesimal square 

 on the surface of a round bar with its sides A C and B D parallel to 

 the axis of the cylinder, before torsion, and A B D' 0' the figure 

 into which this square becomes distorted just before rupture, the 

 diagonal A D has become elongated to the length A D', and the dia- 

 gonal B C has become contracted to the length B C, and that there- 



Fig, 1. 



c c 



^.JD' 



fore there must be maximum tension everywhere, across the spiral 

 of which B C is an infinitely short portion. But the specimens are 

 remarkable as showing in softer or more viscous solids a tendency to 

 break parallel to the surfaces of "shearing" AB, CD, rather than 

 in surfaces inclined to these at an angle of 45°. Through the kind- 

 ness of Mr. Kirkaldy, his specimens of both kinds are now exhibited 

 to the Royal Society. On a smaller scale I have made experiments on 

 round bars of brittle sealing-wax, hardened steel, similar steel tem- 

 pered to various degrees of softness, brass, copper, lead. 



Sealing-wax and hard steel bars exhibited the spiral fracture. All 

 the other bars, without exception, broke as Mr. Kirkaldy's soft steel 

 bars, right across, in a plane perpendicular to the axis of the bar. 

 These experiments were conducted by Mr. Walter Deed and Mr. 

 Adam Logan in the Physical Laboratory of the University of Glas- 

 gow ; and specimens of the bars exhibiting the two kinds of fracture 

 are sent to the Royal Society along with this statement. I also 

 send photographs exhibiting the spiral fracture of a hard steel cylin- 

 der, and the "shearing" fracture of a lead cylinder by torsion. 



These experiments demonstrate that continued " shearing " pa- 

 rallel to one of planes, of a viscous solid, developes in it a ten- 

 dency to break more easily parallel to these planes than in other di- 

 rections, or that a viscous solid, at first isotropic, acquires " cleavage- 

 planes " parallel to the planes of shearing. Thus, if C D and A B 



