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BIOLOGICAL STUDIES ON THE BACTERIA OF THE MOUTH. 99

gas often enough escapes as soon as an opening is made into
a piilp-chamber containing a gangrenous pulp. Xo one, how-
ever, as tar as I am aware, has observed that the air enters the
pulp-eliamlK'r under such conditions.
It would indeed be difficult to explain how a vacuum could
exist in the canal of a root. Granted, however, that a partial
vacuum did exist at the time the pulp-chamber was opened, and
that ten cubic millimeters of air were admitted into the pulp-
chamber (which is a large estimate), how many organisms would
likely be introduced wirh it ?
The number of micro-organisms in a given quantity of air
depends of course upon the purity of the air, and consequently
varies according to the locality. Pure air in Berlin was found
to contain, on the average, 0.1 to 0.5 bacteria per liter, and the
air in hospital wards, with seventeen or eighteen beds, to contain
2.4 and 2.7 per liter respectively.
Xow, if we suppose the air of a dental office to contain double
the amount represented by the largest number found in }iure
Berlin air, there will exist in one li'er, that is, in one million
cul)ic millimeters, one germ. Ten cubic millimeters would then
contain iqooooo X 10 = touW germs.
The chances would then be that in one hundred thousand
operations of this kind the success of one would be endangered
by the entrance of one germ into the pulp-ca^^ty. AVhether this
one germ in such case could produce any disturbance, would
depend upon the existing circumstances.
The appearance of pericementitis after opening into a pulp-
chamber is due solely to the carelessness or helplessness of the
operator. He either forces particles of the putrid pulp through
the foramen apicale, or introduces new infected fermentable mat-
ter into the root-canal from without, or occasions an infection by
means of unclean instruments.
The Relation of Mouth-Bacteria to the Formation of Tartar.
Since the discovery of the existence of great numbers of
microscopical organisms in the hunuin mouth, repeated efforts
have been made to make them rcsponsil)le for the formation of
tartar. Lel)caume compared tartar to coral ; Mandl supposed it