129 
Copper . 
through the whole series, till it is reduced to the thickness 
wanted, the length being increased in proportion as the 
bulk diminishes. 
The copper, after receiving its proper form in the flat- 
ting mills, and cooling, is of a dusky black, or iron co- 
lour, and in order to communicate to it that lively hue 
which is commonly understood to be the true complexion 
of this metal, the plate or bar is heated again for the last 
time in a furnace, and when red hot is plunged into a re- 
cess filled with a saline liquor,* where it assumes that 
colour in a few moments, and being withdrawn, the cop- 
per is put aside as being finished for exportation* 
[3 Archives^ 14„ 
To Separate Copper from Silver . 
S ir— I intended to have deferred the present communi- 
cation till such time as I should have it in my power to 
lay before the public the complete series of experiments 
in which I have been engaged with regard to the purifica- 
tion of gold and silver. But unluckily I mentioned a few 
particular circumstances with regard to them, to a man 
who took it upon him, without my knowledge, to send an 
account of them for publication to a periodical work. As 
I understand that work will not appear so soon as your 
next number, I beg, if you think it worthy of a place, that 
you will insert the following account of some attempts I 
have been making to purify the precious metals. 
Being much at a loss for the want of a crucible of pure 
silver for the analysis of some minerals, and as all the 
usual methods practised for purifying that metal are very 
troublesome, I set myself to consider the various opera- 
tions on metals, in hopes of falling on a more simple way 
* In Aikin’s Chemical Dictionary, it is stated, that this fluid is 
mine. The redness which the copper thus acquires, is consider- 
ed by the merchant as one mark of the purity of the metal. 
Vol. HI. R 
