ALLOY 95 



When there is a strong affinity between tho two metals, their alloy is generally 

 denser than tho mean, and vice versa. This is exemplified, as previously shown, in 

 the alloys of copper with zinc and tin, on tho one hand, and with copper and lead 

 on the other. When one of the metals, however, is added in excess, there result :\n 

 atomic compound and an indefinite combination, as would appear from Muschcnbroek's 

 experiments. Thus : 



1 of lead with 4 of silver give a density of 10-480 

 1 do 2 do 11-032 



1 do 3 do 10-831 



The proportion of the constituents is on this principle estimated in Prance by tho test 

 of the ball applied to pewter ; in which the weight of the alloyed ball is compared with 

 that of a ball of pure tin or standard pewter cast in the same mould. Alloys possess 

 the elasticity belonging to the mean of their constituents, and also tho specific heat. 



According to M. Rudberg, while lead solidifies at 325 C., and tin at 228, their 

 atomic alloy solidifies at 187, which he calls the fixed point, for a compound Pb Sn 3 . 



An alloy too slowly cooled is often apt to favour the crystallisation of one or moro 

 of its components, and thus to render it brittle ; and hence an iron mould is preferable 

 to one of sand when there is danger of such a result. 



It is not a matter of indifference in what order the metals are melted together in 

 making an alloy. Thus, if we combine 90 parts of tin and 10 of copper, and to this 

 alloy add 10 of antimony; or if we combine 10 parts of antimony with 10 of copper, 

 and add to that alloy 90 parts of tin, we shall have two alloys chemically the same ; and 

 still it will be easy to discover thkt, in other respects fusibility, tenacity, &c. they 

 totally differ. Whence this result ? Obviously from the nature of their combination, 

 dependent upon the order pursued in tho preparation, and which continues after tho 

 mixture. In the alloys of lead and antimony also, if the heat bo raised in combining 

 tho two metals together much above their fusing points, the alloy becomes harsh and 

 brittle ; probably because some alloy formed at that high temperature is not soluble 

 in the mass. 



In common cases the specific gravity affords a good criterion whereby to judge of 

 the proportion of two metals in an alloy. But a very fallacious rule has been given 

 in some respectable works for computing the specific gravity that should result from 

 the alloying of given quantities of two metals of known densities, supposing no 

 chemical condensation or expansion of volume to take place. Thus, it has been taught, 

 that if gold and copper be united in equal weights, the computed specific gravity is 

 merely the arithmetical mean between the numbers denoting the two specific gravities. 

 Whereas, the specific gravity of any alloy must be computed by dividing the sum of 

 the two weights by tho sum of the two volumes, compared, for convenience sake, to 

 water reckoned unity. Or, in another form, the rule may be stated thus : Multiply 

 the sum of the weights into the products of the two specific-gravity numbers for a 

 numerator ; and multiply each specific-gravity number into tho weight of the other 

 body, and add the two products together for a denominator. The quotient obtained 

 by dividing the said numerator by the denominator is the truly computed mean 

 specific gravity of the alloy. On comparing with that density tho density found by 

 experiment, wo shall see whether expansion or condensation of volume has attended 

 the metallic combination. Gold having a specific gravity of 19-36, and copper of 8'87, 

 when they are alloyed in equal weights, give, by the fallacious rule of the arithmetical 



-- * 



mean of the densities - - =14-11 ; whereas the rightly computed density is 



II 



only 12-16. It is evident that, on comparing the first result with experiment, wo 

 should be led to infer that there had been a prodigious condensation of volume, though 

 expansion has actually taken place. Let W, w be the two weights ; P, p the two 

 specific gravities, then M, the mean specific gravity, is given by the formula 



2 . _( 



Pw + pW P+p 



= twice the error of the arithmetical mean ; which is therefore always in excess. 



Alloys of a somewhat complex character are made by Mr. Alexander Parkos, of 

 Birmingham, of a white or pale colour, by melting together 33 Ibs. of foreign zinc, 

 64 of tin, 1^ of iron, and 3 of copper ; or 50 zinc, 48 tin, 1 iron, and 3 copper ; or any 

 intermediate proportion of zinc and copper may bo used. The iron and copper are 

 first melted together in a crucible, the tin is next introduced, in such quantities at a 

 time as not to solidify tho iron and copper ; tho zinc is added lastly, and tho whole 

 mixed by stirring. Tho flux recommended for this alloy is, 1 part of lime, 1 part of 

 Cumberland iron ore, and 3 parts of sal-ammoniac. 



