1906.] on the Internal Architecture of Metals. 253 



possessing an almost glassy brittleness. Therefore, the metal bismuth 

 must act indirectly, its presence determining the maintenance of the 

 molecules of gold in a brittle allotropic moditiciition. 



In 1896 there was published in "Engineering" from the labora- 

 tories of the Sheffield College an unambitious research recording the 

 discovery of eutectic cements, which to a considerable extent altered 

 the whole trend of metallurgical thought. 



Fig. 2 shows a micro-section of the structure of gold to which * 2 

 per cent, of bismuth had been added. The microscope had at once 

 explained the hitherto mysterious action of bismuth. It indicated 

 clearly that the small quantity of bismuth alloyed with a definite 

 amount of gold forming a constituent having a much lower freezing 

 point than the main mass. Hence, when crystallisation set in during 

 solidification from a series of centres, the " eutectic " or constituent 

 last fluid was expelled to the exterior of each crystalline grain of pure 

 gold, thus enveloping each crystal in a membrane of gold-bismuth 

 alloy having a much higher co-efficient of contraction than the 

 crystal itself. Hence, during cooling, the gold-bismuth alloy which 

 may be regarded as the mortar of the structure, to a considerable 

 extent detached itself from the crystalline grains of gold which may 

 be regarded as the stones of which the mass is built up. In the 

 micrograph. Fig. 2, the stones of tough gold are represented as 

 white, whilst the mortar of gold-bismuth eutectic is shown as dark, 

 thick, enveloping membranes. These membranes become pasty well 

 below a red heat, and it was proved that at 400° C. the mass could l)e 

 powdered in a mortar, the crystalline grains of pure gold becoming 

 detached from the feeble alloy cementing them together. One 

 of these crystalline grains exhibited no signs of the brittleness of 

 the mass from which it Avas thus detached, but readily beat out into 

 gold leaf in the ordinary manner. 



Passing ^'om gold to brass, it was proposed to diverge from the 

 abstract to the concrete, and to show the value of the appHcation of 

 the science of metallurgy to practical problems connected with 

 mysterious failures in marine engineering. 



A notable case in point was the explosion of the brazed copper 

 main steam-pipe of the s.s. " Prodano " in calm weather off the 

 Kentish Knock at a pressure about one-tenth of that to which it had 

 been previously tested. In this case the microscope was again 

 successful in clearly indicating the nature of the electrolytic decay 

 under certain conditions of brass used in naval architecture. In this 

 connection, a familiar phenomenon is the decay of Muntz metal bolts 

 exposed to the action of bilge water. Such bolts break suddenly and 

 present a distinctly coppery fracture. A micrographic examination 

 of such bolts usually revealed a minor area of undeteriorated brass 

 and a major area of deteriorated brass— that was to say, brass which 

 had been more or less dezincified, an expression which meant, in other 

 words, that the mass had become transformed into rotten, spongy 

 copper. 



