158 



Iron. 



Contraftion by 

 tin, bifmuth, 

 zinc, and anti- 

 mony. 



Miftake of 

 Briflbn in the 

 alloy of geld 

 with copper. 



Specific gravity 

 of a mixed metal 

 affefted by vari- 

 ous circum- 

 ftances. 

 Differences in 

 the fame bar. 

 Whence. 



Unequal diffu- 

 fion of alloy. 



Mixed metals 

 feparate under 

 fufion. 



Long continued 

 fiidion leffens 

 fpecific gravity 

 of metals. 

 True chemical 

 combinations. 



EXPERIMENTS ON VARIOUS ALLOY? OF GOLD. 



per the expanfion was 0,67 ; with iron it was 0,44 ; with iron 

 and copper 0,37. Tin, bifmuth, zinc, and antimony, pro- 

 duced a contradlion. Lead and bifmuth very much refemble 

 each other in their effects on gold, and in the irregularity of 

 thefe in various proportions. 



Mr. Briflbn has obferved, that on alloying gold with .j*^ of 

 copper, a mutual penetration takes place ; but in the courfe of 

 the prefent experiments the reverfe of this was found. It is 

 probable, therefore, that his experiment was made with part 

 of a large bar or ingot ; as the unequal diffufion of the alloy, 

 the quantity of the metal, and the nature, form and pofition 

 of the mould, are all capable of affefting the fpecific gravity. 

 Thus, when the mixture is perfect, the bottom of a bar caft 

 in a vertical mould will be of the greateft fpecific gravity, 

 owing to the preflTure of the fuperincumbent metal, while the 

 quality of both ends appears equal by the aflay. On the con- 

 trary, if the mixture be imperfect, the lighter metal flowing 

 out of the crucible firft, will render the bottom of the bar in- 

 ferior both in quality and in fpecific gravity, as was found by 

 experiment. 



This unequal diffufion of the alloy through the raafs, the 

 exa6l diftribution of which is not fo eafy as may be imagined, 

 particularly in large quantities, is the moft frequent caufe of 

 the variation in the fpecific gravity of ftandard gold. The dif- 

 ficulty has been confidered, and an allowance made for it, 

 called the remedy for the mafter of the mint. Even when 

 metals have been completely mixed, if they be kept in fufion 

 under certain circumftances, afeparation, more or lefs perfedt; 

 fometimes takes place. This reparation appears to be accord- 

 ing to the relative affinities and fpecific gravities of the two 

 metals, and is the fooneft effe6led when the metals have not 

 been perfedly mixed *. 



Befidc the caufes mentioned there is another, that occafions 

 variations more or lefs confiderable in the fpecific gravity of 



* Some compound metals may perhaps be mere mechanical mix- 

 tures j but lam inclined to believe, that by much the greateft num- 

 ber are true chemical combinations j and confequently, when thefe 

 laft have been properly formed, a reparation of the component 

 metals, by the means above-mentioned, can feldom if ever be ef- 

 feaed. C. H. 



metals. 



