ALUM. MAGNESIA . 12J 
ration of butter. It has a tendency to retard igni- 
tion. Paper soaked in alum water does not easily take 
fire, and is thereby better fitted for the preservation of 
gunpowder. Such paper is likewise used in the whiten- 
ing of silver, and the silvering of brass. It has been 
recommended that ladies' muslin dresses should be 
dipped in a solution of this substance, for the purpose 
of rendering them less liable to catch tire. A solution 
of alum also retards the putrefaction of animal sub- 
stances, and affords useful, as well as economical, 
means of preserving natural productions that are im- 
ported from foreign countries. Alum is frequently 
mixed with paste, to prevent its losing its tenacity by 
the absorption of moisture. It is asserted that bakers 
occasionally use it as an ingredient in bread, and that 
its presence may be discovered by thrusting a heated 
knife into a loaf before it is cold: if free from alum, 
scarcely any alteration will be visible on the blade, but 
if the contrary, the surface, when cool, will appear 
slightly covered with an incrustation of alum. A very 
important purpose to which alum may be applied is in 
the purifying 'and sweetening of water that has become 
fetid and unfit for use ; from five to ten grains of burned 
alum, and double or treble that quantity of pounded 
charcoal, wiil correct the fetor of a gallon of water. 
Printers' cushions, and the blocks used for the printing 
of calicos, are rubbed with burned alum to remove 
any greasiness, which otherwise would prevent the ink 
or colour from sticking. This substance is also occa- 
sionally employed by surgeons to stop the bleeding of 
small vessels, to corrode fungous or proud flesh, and 
for other purposes in medicine. 
198. MAGNESIA FAMILY. 
This is a family of minerals which comprehends all the 
combinations of magnesia with acids. 
When freed from extraneous matters, magnesia is a pow- 
dery substance of limpid white colour. 
