252 Professor John Oliver Arnold [Feb. 23, 



WEEKLY EVENING MEETING, 



Friday, February 23, 11)06. 



IIis Grace the Duke of Northumberland, K.G. P.O. D.C.L. 

 F.R.S., President, in the Chair. 



Professor John Oliver Arnold, Professor of Metallurgy, 

 Sheffield University. 



The Internal Architecture of Metals. 



[abstract.] 



It had been cynically remarked that to deliver a successful scientific 

 lecture to a cultured audience it was necessary to divide the lecture 

 into three parts. The first part should be understood both by the 

 audience and the lecturer ; the second part by the lecturer and not 

 by the audience ; and the third part neither by the audience nor the 

 lecturer. 



If the foregoing dictum were true, the speaker found himself in a 

 paradoxical position. The object of the discourse was to make the 

 subject under consideration as clear as possible throughout, hence the 

 more nearly this object was achieved, the more unsuccessful the 

 lecture. The title of the discourse might seem to some far-fetched, 

 since, superficially, a bar of polished brass or steel apparently pre- 

 sented the arch type of a homogeneous solid. Any such idea, 

 however, must in a few moments be dispelled. Taking a section of 

 ])ure gold, or at any rate of gold of a purity of 99-995 per cent., 

 this, when polished and etched, presented under a low power of 

 the microscope large allotrimorphic crystals, the etching figures of 

 which exhibited varying orientation in different crystals. Hence (see 

 Fig. 1) one crystal might appear black, another show the brilliant 

 yellow of gold, and a third exhibit middle tone. All these were 

 purely optical effects. In the black crystal the orientation was at 

 such an angle as to reflect the light entirely outside the objective, 

 whilst, going to the other extreme, the gold-coloured crystal had a 

 molecular orientation which reflected the light entirely into the objec- 

 tive. It was well known that the addition of one or two tenths per 

 cent, to gold of the metal bisnmth produced a surprising mass 

 brittleness which naturally led to the enunciation of theories 'to 

 account for so remarkable a phenomenon. 



Twelve years ago the theory wliich commanded a general accept- 

 ance, and at that time reasonably so, was that the small quantity of 

 bismuth was incapable per se of producing so profound a mechanical 

 change as to convert one of the most ductile of metals into a mass 



