ftlNUTE DIVISION OP MERCURY. 
241 
sebate of mercury, with a portion generally of metallic mercu- 
ry, its activity, of course, depending on the former." 
This explanation of the oxidation of mercury by grease is 
not very conclusive, since we know, that by means of 
saccharine substances, which are unfavorable to oxidation, 
the metal is reduced to the same condition in which it exists 
in the ointment. If unctuous substances possess the general 
oxidizing property alluded to by Mr. Murray, it is difficult 
to account for their successful application to the protection of 
machinery and metallic implements from rust. 
(Duncan, 1815. J — " Quicksilver has a strong affinity for 
oxygen, and absorbs it slowly from the atmosphere." * * 
" The black oxide is the mildest, but, at the same time, the 
most efficacious of the preparations- of mercury. Combined 
with magnesia or chalk, it is not in general use ; but in the 
form of the common mercurial pill and ointment, it is more 
employed than any other preparation of the same metal, ex- 
cept calomel." 
This statement 'is contradicted by Phillips in the Pharmaco- 
poeia of 1824. 
Hyd. cum Creta. ( Phillips' Pharmacopoeia, of 1S24.^ 
"I have only slightly examined this preparation, and I am 
uncertain whether it consists merely of chalk and mercury, 
in a state of minute division, or whether it is $ sub-oxide of 
mercury formed by absorbing oxygen during the trituration." 
44 The mercury is totally insoluble in acetic acid, and, 
therefore, is not the black or protoxide ; but whetv the chalk 
has been separated by acetic acid, the mercury does not form 
one fluid mass, like metallic mercury, but exists in the state 
of separate and minute globules." 
Pil. Hydrarg, " It has been asserted, that the mercurial 
pill and ointment both contain the black or protoxide of mer- 
cury. It is, however, possible, as I have already hinted, 
when treating of hydrarg. cum creta, that a sub-oxide of mer- 
cury may exist, and form the base of these preparations." 
Rennie, in the year 1829, differs from Phillips. 
Hyd. cum Creta. (Bennie's Supplement, 1829. J— 
