116 
ON HYDRARGYRUM CUM CRETA. 
Theoretically, the powder should have contained 37.5 
per cent of mercury. The deficit of 1.37 per cent must be 
attributed to loss in the preparation of the powder origi- 
nally, and inaccuracy in the analysis. 
It is a matter of speculation what should have caused this 
change in the preparation. The powder was prepared by tri- 
turating 6 drachms of resin (colophony) and 3 oz. of mercury, 
together with sufficient alcohol added from time to time to 
keep the mixture of a pasty consistence, until the mercury 
was extinguished properly. Five ounces of prepared chalk 
was then gradually added, (the consistence being kept 
pasty) until the whole was thoroughly mixed. The resin 
was then removed by washing with alcohol — allowing the 
powder to subside, and decanting, several times renewed 
and finally lixiviated on a filter with the same fluid until 
the alcohol that passed ceased to communicate opalescence 
to water. Can the resinous acid (colopholic acid) of the 
rosin possess the property of inducing oxidation ? and does 
this agent, by enabling the operator to push the division of 
the mercury to a very high degree, rendering the powder 
extremely porous, so extend its surface as to give it a ten- 
dency to absorb and concentrate oxygen from the air in its 
interstices, as is the case with platina sponge, and iron by 
hydrogen? In the several preparations in which mercury 
exists in a minute state of division, various suggestions have 
been made with a view of saving labor, especially in mak- 
ing mercurial ointment. These suggestions are of two kinds 
— those which act chemically and those whose influence 
may be considered mechanical. Sulphur, oil of egg, and bal- 
sam of sulphur belong to the first class ; and when used their 
action is due to the formation of an exceedingly minute 
coating of sulphuret of mercury on the surface of the glo- 
bules, which when divided are thereby prevented from 
coalescing. Rancid fatty matter, and resins, both of which 
possess this power of facilitating the extinguishment of mer- 
cury in an eminent degree, I have heretofore looked upon 
