July i6, 1896 J 



NA TURE 



257 



interest in the subject has however arisen in connection with 

 the extraordinary development in various parts of the world, and 

 especially in South Africa, of certain processes which are now 

 employed for extracting gold from its ores. Their use has 

 been attended with the introduction into this country of a series 

 of alloys of gold and of the base metals, which have hitherto 

 rarely been met with in metallurgical industry. The base metals 

 associated with the gold in these cases are usually the very 

 ordinary ones lead and zinc ; but their presence in the gold has 

 given rise to unexpected difficulties, as the distribution of the 

 precious metals in the ingots which now reach this country is so 

 peculiar, that it is not possible to estimate the value of the ingots 

 by taking the pieces of metal required for assay, by any of the 

 well-known methods at present in use. 



Investigation of the cause of the singular molecular arrange- 

 ment of the ingots, has revealed many facts of scientific as well 

 as industrial interest, which the author describes at length. The 

 following case of an ingot of gold may be taken as typical. 



\ 



I'our a'^says were made on a portion of metal cut from the points 

 marked a, at the top of the ignot : the highest of the results of 

 a.ssay indicated that 664 parts of gold were present in looo parts 

 of the alloy, while the lowest assay gave only 465 parts. On 

 the other hand, three assays on a piece of metal cut from the 

 bottom of the ingot, at b, gave 652 parts of gold in 1000 as the 

 highest, and 332 '5 as the lowest. Clearly, therefore, the action of 

 gravity does not explain the distribution of the precious metal. 



The ordinary course, where divergent results of assay are 

 obtained on portions of metal cut from such an ingot, would be 

 to melt the entire mass, and take a " dip " assay piece, that is, to 

 remove a portion of metal from the well-stirred fluid mass. This 

 was done in all the cases cited in the paper here abridged, and as 

 regards the mass of gold to which reference has just been made, 

 assays on the porlion of metal removed from the fluid mass gave 

 results which were still very conflicting, the lowest assay showing 

 the presence of 562-3 parts of gold, and the highest 653'5. It 

 was evident therefore, that rearrangement could take place 

 within the limits of a fragment of metal which did not weigh 

 more than a few grammes. 



The only method of ascertaining the value of the ingot con- 

 sisted in separating the precious and base metals in mass, and 

 the result of this operation showed the value of the ingot to be 

 ^^1028, while the value, as calculated from the average of the 

 assays previously made, would only have been £<)6$, or a differ- 

 ence :n value of ^63 on an ingot weighing 1 2 223 kilograms. 

 The importance of the question from an industrial point of view 

 will at once be recognised when it is remembered that gold to 

 the value of many millions sterling of the quality represented by 

 the above results, now reaches this country annually. 



Coming now to the scientific side of the problem, analysis of 

 the ingot, to which reference has been made above, showed that 

 it contained the following metals in addition to gold : 



.Silver... 

 Lead ... 

 Zinc ... 

 Copper 

 Iron ... 



S'l per cent. 

 1 6 '4 , , 

 9-5 

 40 



Suspicion at once fell on the lead and zinc as disturbing ele- 

 ments, and their influence was sy.stematically investigated by a 

 lengthy series of experiments, in the course of which gold alloys, 

 containing different proportions of gold and of impurities, were 

 cast in spherical moulds two and three inches in diameter, the 

 solidified masses being explored by assays made on metal repre- 

 senting all parts of the ma.ss. The general result of these ex- 

 periments was to show that lead exerts a greater disturbing 

 influence than zinc. The problem was then attacked from 

 a different point of view. I availed myself of Roberts- 

 .\usten's method of fixing the solidifying points of metals on 



NO. 1394, VOL. 54] 



" cooling curves" obtained by the aid of thermo-junctions con- 

 nected with autographic recorders. Such curves showed that a 

 triple alloy of lead, gold, and zinc has three " freezing points." 

 The mass sets as a whole at a single main point of solidification, 

 but the lead and the zinc associated with some gold retain a 

 certain amount of individual independence, and by falling out of 

 solution, separately destroy the uniformity of the mass, even 

 though the mass itself be small. 



After a long series of experiments, a metallic solvent which 

 would enter into union with the gold, the zinc, and the lead 

 was sought. Silver proved to be such a solvent, and solidified 

 alloys of gold containing not more than 30 per cent, of lead and 

 of zinc, may be made practically uniform in composition by 

 adding 15 per cent, of silver to the mass when fluid. The 

 result is singular, as it shows that there are cases in which the 

 uniformity of a gold alloy may be improved by lowering its 

 standard fineness ; and another proof of .scientific interest is 

 afforded of the fact that alloys behave like saline solutions. 



VlG. I. — Gold 700 parts, lead 300 parts ; weight about two kilograms. 



The result shows a decided tendency of the gold to liqital:- 

 to Ike centre of the mass. 



Fig. 2.— Gold 75 parts, lead 15 parts, j\ 

 kilograms. 



10 parts ; weight about two 



There is evidence of rearrangement by liquation in this case 

 which sends gold to the centre, but the result is complicated, as 

 gravity appears also to send gold to the lower portion of the 

 spherical mass. 



Fk;. 5. — Gold 95 parts. 



ght 4*450 kilograms. 



A slight but decided tendency of liquation of gold towards 

 the centre. 



