142 Remarks on Priestley’s Analysis 
comes of a red colour ; and is then the calx mention- 
ed above. It is probable therefore (and it is allow- 
ed) that this calx still contains sulphate of mercury ; 
which can hardly be expected to be decomposed but 
by adding substances which have a greater, affinity 
to the sulphuric acid than mercury, or else a greater 
attraction to the oxygen of the sulphuric acid than 
the sulphur. However, Dr. Gren, professor at 
Halle, who, in matters of fact, may be depended 
on, informs us (in his Treatise on Chemistry, §. 
2262) that this calx may be reduced to its metallic 
state by mere heat, without any addition whatever ; 
‘and that, during the process, oxygenous gas and vi- 
triolic-acid-air are produced. 
In farther support of his assertion (that mercury, in 
becoming precipitate fer se, parts with none or little 
of its phlogiston) Dr. Priestley says, “ if we judge 
by the air expelled from the calces of metals and 
other circumstances, there are few, if any, of them 
but contain more or less of phlogiston.”—I ima- 
gine he alludes here to the azote, which is mixed 
with the oxygenous gas obtained from the oxyds of 
metals. If this be occasioned by a portion of phlo- 
giston retained by the calx, how does it happen that 
the purest oxygenous gas that can be procured from 
metallic oxyds, is obtained from precipitate per se, 
though, according to Dr. Priestley, it retains all or 
nearly all its phlogiston?—This argument, more- 
over, involves a contradiction to the doctrine of 
