398 Professor W. Chandler Bdberts- Austen [March 26, 



[A mass of inire lead cast into tlie shai^e shown in the sketch, 

 Fig. 2, was struck with a piece of lead, and it emitted a sharp, clear 



note.] /. 1 • 



I have shown you the experiment mainly for the sake of being 



able to quote Eeaumur's observations upon 



it. He showed that the sonorous lead 



'^^^^^ might be rendered dull by hammering it. 



I. .ZZ'^^g^tZ^.:^.:^SP{ Here is lead from the same samj)le of metal 



as that from which the sonorous mass was 

 cast, but it has been flattened out, and you will observe that it is 

 " dull." I think his remarks have been overlooked in late years. He 

 was led to the belief that in cast lead there must be an arrange- 

 ment of the interior of the mass which the hammer cannot impart, 

 because lead fashioned by hammering into the same form as the 

 sonorous cast mass, is dull, and, more important still, he held that the 

 fibrous and granular structure of the lead is modified in a manner 

 which makes it probable that the sound is due to the shape of the 

 grains, and to the " way in which they touch each other " ; 

 further, the blows of the hammer not only change the arrange- 

 ment of the fibres, but they alter the shape of the grains, "the 

 round grains are rendered flat, they are comj)elled to elongate and 

 fill the interstitial spaces which previously existed between them. 

 The particles are no longer free to vibrate, hence the lead is dull." 

 These remarks derive additional interest, if we compare them with 

 the observations in Professor Osborne Keynolds' most important lec- 

 ture on " Dilatancy in Granular Matter " recently delivered here. 

 We shall also, I think, see that this description of Eeaumur's shows 

 that he fully appreciated the theoretical importance of the kind of 

 facts depending on the transfer of metallic matter from one position 

 to another, which we now consider to be characteristic of the 

 " flow " of metals ; at any rate I have thought it well to make 

 L emery's experiment the starting point of the rest of the remarks 

 I have to offer you. 



A solid may be very brittle, and may yet, if time be given to it, 

 flow from one point to another. This stick of sealing-wax was sup- 

 ported at its ends, and it has in a few weeks bent at the ordinary 

 atmosj)heric temperature, although at any given point of its flow it 

 would have been easy to snap it with a slight application of force. 

 This much thinner strip of pure lead of the same breadth as the 

 sealing-wax, also bends at the ordinary temperature with its own 

 weight, the ends being supported. Sir William Thomson has 

 pointed out that a gold wire sustaining half the weight which would 

 actually break it, would probably not rupture in a thousand or 

 even a million years, that is to say, there would be no " flow " 

 ending in disruption ; if, however, force be suitably applied, metals 

 will flow readily. First, let us examine the case of a metal under 

 force applied so as to compel it to flow through a hole, and I would 

 point to the analogy of an ordinary viscous fluid. This vessel con- 



