ON HYDRARGYRUM CUM CRETA. 
117 
as acting mechanically by their ability to cling to or stain 
the bright surfaces of the globules of mercury, and thus pre- 
vent their coalescence when once divided. Now, on re- 
flection, I am inclined to believe that these substances may 
act by slightly oxidizing as well as staining the minute par- 
ticles of mercury, and commence a process which is subse- 
quently carried on by the oxygen of the air. In the case of 
blue mass, the sugar is well known to act as a preventive 
of oxidation, and hence that preparation ought to be less 
liable to change by atmospheric exposure than the others. 
Whatever may be the cause of this extraordinary change, 
the above results lead us to reject the use of resin, as in the 
process of Stewart, or any other labor-saving agent having 
the same tendency, and I think it may be worthy o investi- 
gation, whether a mixture of pure chalk and mercury, in 
which the latter is in a state of extreme division, will not 
gradually absorb oxygen and have its properties changed. 
The ordinary glass stoppered bottles, such as the specimen 
analysed was kept in, will not exclude the air, and if they did 
the frequent opening in the process of dispensing will afford 
abundant oppportunity for the production of this change, if 
it really will occur unassisted by any foreign substance. 
