198 _ ESSAYS anp OBSERVATIONS 
ved in acids, and then feparated from 
them again by alkalis, proceeds from air 
furnifhed by the alkalis. And that, in 
the aurum fulminans, which is prepared by 
the fame means, this air adheres to the 
gold in fuch a peculiar manner, that, in a 
moderate degree of heat, the whole of it 
recovers its elafticity in the fame inftant 
of time; and thus, by the violent fhock 
which it gives to the air around, produ- 
ces the loud crack or fulmination of this 
powder. Thofe who will imagine the 
_ explofion of fuch a minute portion of fix- 
ed air, as can refide in the aurum fulminans, 
to be infufficient for the exceffive loudnefs 
of the noife, will confider, that it is not a 
large quantity of motion communicated 
tothe air, but rather a fmart ftroke, which 
produces found, and that the explofion of | 
but a few particles of fixed air may be 
capable of caufing a loud noife, provided 
they all recover their {pring fuddenly, and 
in the fame inftant. | 
‘THE above experiments lead us alfo to 
conclude, that volatile alkalis, and the 
common abforbent earths, which lofe 
their 
