MERCURIAL OINTMENT AND VAPOR OF MERCURY. 
59 
mains to be shown in what other manner this passage takes 
place. 
When mercurial ointment is melted in a test-glass, the metal 
separates from the fat, so that the former constitutes a white layer 
at the bottom of the glass, whilst the latter forms a yellow super- 
nant liquid; whilst between the two there is a thin black stratum, 
which appears thinner when recently prepared ointment is used, 
and deeper when the ointment is old. This is also seen when the 
metal is separated from the fatty matter by solution in ether. 
Hence, in addition to metallic mercury and fatty matter, the oint- 
ment contains a black substance, the quantity of which increases 
with its age ; it is also a well-known fact, that the ointment gra- 
dually gets darker by keeping. It is natural to regard this black 
matter as protoxide of mercury, the formation of which is favored 
by the absorption of oxygen occurring whilst the fatty matter was 
becoming rancid. That this w T as the case should be proved by the 
following experiment. Some freshly prepared ointment was 
treated with ether, the solution poured off ; the metallic residue 
was washed with ether, and then treated with water to which a 
few drops of sulphuric acid had been added. The acid, which was 
cold, and very dilute, could not attack the metallic mercury, but 
would dissolve any oxide which was present. A few drops of so- 
lution of sulphuretted hydrogen were added to the filtered solution, 
when a brown turbidity immediately appeared, which after some 
hours had subsided in the form offtakes. When, instead of the fresh 
ointment, some of which had been longer kept was used, the same 
reaction appeared much more strongly, and a distinct precipitate of 
sulphuret of mercury was formed. 
In another experiment, acetic acid was substituted for the sul- 
phuric acid. The result was the same. It is thus proved that 
mercurial ointment contains protoxide of mercury in addition to 
metallic mercury and fatty matter.* 
Donovan and Christison long since asserted this to be the case, 
*That finely-divided mercury gradually becomes partially converted into 
oxide is in conformity with Poggendorff's experiments, according to which 
the surface of a perfectly pure mass of mercury soon loses its mobility for 
feeble electrical actions. If the mercury be placed in contact with acid, it 
reobtams its mobility, which it loses, only in gases containing oxygen, not 
in carbonic acid and hydrogen. See Pogg. Annal., lxxvii. p. 9. 
