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10 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.
arise, has not been finally decided. The tendency seems to be
to accept the latter view, although positive evidence for the
origin of new species has not yet been adduced.

5. Life-Conditions of Bacteria.

I designate as life-conditions of bacteria those circumstances
relative to food, air, temperature, etc., which are indispensable
to their development.
Xencki's^ analysis gives for bacteria the following chemical
composition: water 84.81 per cent, albumen 13.03 per cent,,
fat 1.20 per cent., ashes 0.61 per cent., undetermined residue
0.S2 per cent.
xVccording to this analysis a nutrient solution for bacteria
should be composed of albumen, carbohydrates, and small
quantities of salts; a conclusion which has been completely
confirmed by experience, which has taught that such solutions
invariably form the best culture media.
The juices and accumulations in the human mouth at all times
present such a medium. Artificially it may be prepared in dif-
ferent ways, perhaps most easily by means of the following mix-
ture : water 100.0, peptone 2.0, sugar 1.0, beef extract 1.5.
The solid culture media introduced by Koch, which are pre-
pared by the addition of gelatine or agar-agar, are similarly com-
posed.
The composition ami concentration of the culture medium are of
the greatest importance. The organic substances, albumen, pep-
tone, sugar, etc., are without question the most suitable culture
media, although formerly mineral solutions were extensively
used. One of these, recommended by Niigeli, has the following
composition: water 100 c.cm., tartrate of ammonia 1.0, phos-
phate of potassium 0.1, sulphate of magnesium 0.02, chloride of
calcium 0.01. Such mineral solutions are, however, as a rule,
by no means so well adapted to culture experiments as the media
now in use.
For each species of bacteria there seems to be a certain con-
centration of the culture medium best suited to its growth,
althouo-h it has not as yet been possil)le to establish definite laws