defibrinated blood and thin it by adding an equal amount of water. Then
place the hand over the mouth and shake until the blood is thoroughly
mixed with the air. Compare with a portion of the blood not mixed with the
air, noting any difference in color. What substance in the air has acted
on the blood to change its color?

3. Fill three tumblers each two thirds full of water and set them in a
warm place. Pour into one of the tumblers, and thoroughly mix with the
water, two tablespoonfuls of the blood containing the Epsom salts. After
an interval of half an hour add blood to the second tumbler in the same
manner, and after another half hour add blood to the third. The water
dilutes the salts so that coagulation is no longer prevented. Jar the
vessel occasionally as coagulation proceeds; and if the clot is slow in
forming, add a trace of some salt of calcium (calcium chloride). After the
blood has been added to the last tumbler make a comparative study of all.
Note that coagulation begins in all parts of the liquid at the same time
and that, as the process goes on, the clot shrinks and is drawn toward the
center.

4. Place a clot from one of the tumblers in experiment 3 in a large vessel
of water. Thoroughly wash, adding fresh water, until a white, stringy
solid remains. This substance is fibrin.

5. Examine the coagulated blood obtained from the butcher (bottle 2).
Observe the dark central mass (the clot) surrounded by a clear liquid (the
serum). Sketch the vessel and its contents, showing and naming the parts
into which the blood separates by coagulation.

*To examine the Red Corpuscles.*—Blood for this purpose is easily obtained
from the finger. With a handkerchief, wrap one of the fingers of the left