330 Dr. Priestley’s Experiments and Obfervations , &c. 
of common fait and of fal ammoniac. In a few days the fal 
ammoniac was found to have converted the copper into a dry 
green calx ; but the common fait adted more {lowly upon the 
copper, and continued moift during feveral weeks. At length, 
however, at the end of two months, a ‘dry green calx was 
formed, and I obferved, that round the edges of the calx 
where it joined to the furtace of the copper not adled upon, 
and where the liquor had la ft flood before it was dried, an 
efflorefcence had formed of mild mineral alkali, which feems 
to have been fepa rated from its acid by the adlion of the cop- 
per on the acid, and perhaps aided by the affinity of the alkali 
itfelf to fixed air. 
The fal ammoniac alfo had probably been decompofed by 
the copper, but the volatility of its alkaline bafis, when dif- 
engaged, had prevented its appearance. 
Wiffiing you may long continue to add to the numerous and 
fplendid lift of difcoveries with which you have already en- 
riched natural philofophy, 
l I am, &c. 
JAMES KEIR, 
Ivlarch 26, 1788. 
