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Full text of "Pathogenic Bacteria"

TYPHOID FEVER.                        385

3. The dilution of the known quantity of blood con-
tained in the tube with a measured quantity of the
bouillon, or diluted agar-agar, culture of the bacillus.

The standard tube that I adopted had a diameter
about equal to the E string of a violin. A larger or
smaller tube would have done quite as well. In such a
tube the column of blood rises about an inch and weighs
about 0.018 gram. As personal equation in judging
size is a marked source of error, the experimenter must
work out his own standard tube and not adopt that which
has just been given. It is important to know the length
of the column that has a certain weight, because, as each
tube is not separately measured and graduated, the two
chief means of avoiding error will be (i) to have the
tubes as nearly as possible of equal diameter, and (2) to
prove them to be so by observing that the columns of
fluid they contain when used are of the same length, re-
jecting one after another all the tubes which seem to
the eye to have the proper caliber, but in which the
column is obviously longer or shorter than that of the
original tube.

Keeping the standard tube before him as a guide, and
using a Bunsen flame—which is better than a blowpipe,
because it does not heat the glass so rapidly and make it
so soft—the experimenter prepares one hundred or more
capillary tubes as nearly as possible of the same size as
that of the standardized tube. All the irregular sizes
are rejected, and the suitable sizes cut into portions
about three inches long. These pieces, which should
number several hundred (it is economy to make a large
number at a time), are now carefully sorted, being com-
pared with the standard tube at both ends, and thrown
away if too large or too $mall at either. It is best to
sort the tubes twice on different days, or have several
different persons go over them all. Of course, some
tubes of quite different caliber will, in spite of all pre-
cautions, remain in the bundle, but this is no serious
matter, because at the last moment the height of the

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