1876.] Prof. Hughes, On the Fracture of Flint 13 



He then drew attention to a series of specimens which shewed 

 that when flint or other material of a similar texture was struck 

 by any object such as a round-headed hammer, so that the blow 

 was symmetrically distributed over a small area, a bruise was pro- 

 duced which on weathering flaked off all round a small cone 

 having an angle at its apex of about 110°, and that when the 

 whole had flaked away a small smooth basin was left. But if, and 

 only if, the blow was sufiiciently intense to break the flint up, this 

 cone was found to truncate a larger cone whose apex had an angle 

 of about 80°. 



He pointed out that modifications of this double cone structure 

 explained the " rings " and " bulb of percussion " which were ap- 

 pealed to as evidence of the direction of blows on which arguments 

 were founded as to the origin of some stone implements. 



Professor Stuart pointed out tliat the explanation of the 

 conical fracture shewn by Professor Hughes of the surface-pits was 

 to be sought for in the homogeneous character of the substance. 

 The primary cone bore a strong resemblance to that frequently 

 seen in specimens of iron fractured by pressure ; the cause of the 

 formation of which was the shearing stress called into play by the 

 pressure. According to the usual imperfect theory of elasticity 

 the vertical angle of this cone would be 90", but in iron thus frac- 

 tured by pressure without lateral constraint it was less than that, 

 owing apparently to the tangential action between contiguous 

 layers of the nature of friction, i. e. increasing with the pressure. 

 This action undoubtedly existed in other substances, and the ver- 

 tical angle of the cone in any such fractures as those exhibited by 

 Professor Hughes would thus depend on the direction of greatest 

 pressure called into play by the blow, the circumstances being 

 those of lateral restraint. Professor Stuart exhibited a drawing of 

 a piece of oak which had been subject to great pressure, and which 

 shewed by the condition of distortion of its fibres the planes of 

 greatest shearing stress, inclined at an angle of about 110° in the 

 permanently distorted piece of oak. He proceeded to shew that 

 the texture of the primary cone would be in general destroyed, so 

 that in process of time it would be weathered oft", leaving the pits 

 pointed out by Professor Hughes. — As to the secondary cone, he 

 also distinguished the similarity which it bore to the form of 

 a piece of iron punched out by a die resembling the head of the 

 hammer by which the fracture in the flint in question was caused. 

 — The irregular conchoidal fractures were due to the reflection of 

 the wave produced by the impact from the various bounding sur- 

 faces of the specimen of flint. 



