luid drawn into
the stem from the next succeeding one.

9. Again introduce the end of the pipette into the fluid and draw up a
second volume of saline to the level of the grease pencil mark, and
follow this with a second air index.

10. In like manner take up seven more equal volumes of saline solution
and their following air bubbles. There are now nine equal volumes of
normal saline in the pipette.

11. Now pass the point of the pipette into the blood tube and dip the
open end below the surface of the serum. Proceeding as before, aspirate
a volume of serum into the capillary stem up to the level of the pencil
mark.

12. Eject the contents of the pipette into the small tube marked 10 per
cent. by compressing the rubber teat between thumb and finger.

13. Mix the one volume of serum with the nine volumes of saline solution
very thoroughly by repeatedly drawing up the whole of the fluid into the
pipette and driving it out again into the test-tube.

14. Now take a clean pipette and proceed precisely as before, 4 to 10.

15. Having aspirated nine equal volumes of saline into this second
pipette, now take up one similar volume of the fluid in the "10 per
cent. tube."

16. Eject the contents of this pipette into the second tube marked 1 per
cent. and mix thoroughly as before.

17. In similar fashion make the 0.1 per cent. solution and transfer to
the third tube.

18. Further dilutions in multiples of ten can be prepared in the same
way, and by varying the number of volumes of diluting fluid or serum any
required dilution can be made (see Appendix, Dilution Tables).

NOTE.--The saline diluting fluid _must always_ be taken into
the pipette first, otherwise if the serum contains a very
large amount of agglutinin the traces of this serum added to
the saline solution may be sufficient to entirely vitiate
the subsequent observations--whilst if more than one sample
of serum is diluted from the same saline solution serious
errors may be introduced into the experiments.

~The Microscopical Reaction:~

_Apparatus Required:_

Five hanging-drop slides (or preferably two slide), with two
cells mounted side by side on each (Fig. 62, a), and one
slide with one cell only.

Vaseline.

Cover-slips.

Platinum loop.

Grease pencil.

Eighteen to twenty-four-hour-old bouillon cultivation of the
organism to be tested (e. g., Bacillus typhi a