Experiment 67.  Make a thick starch paste; place some in test tubes, labeled A and B.  Keep A for comparison, and to B add saliva, and expose both to about 104 degrees F. A is unaffected, while B soon becomes fluid—­within two minutes—­and loses its opalescence; this liquefaction is a process quite antecedent to the saccharifying process which follows.

Experiment 68. To show the action of gastric juice on milk.  Mix two teaspoonfuls of fresh milk in a test tube with a few drops of neutral artificial gastric juice;[30] keep at about 100 degrees F. In a short time the milk curdles, so that the tube can be inverted without the curd falling out.  By and by whey is squeezed out of the clot.  The curdling of milk by the rennet ferment present in the gastric juice, is quite different from that produced by the “souring of milk,” or by the precipitation of caseinogen by acids.  Here the casein (carrying with it most of the fats) is precipitated in a neutral fluid.

Experiment 69.  To the test tube in the preceding experiment, add two teaspoonfuls of dilute hydrochloric acid, and keep at 100 degrees F. for two hours.  The pepsin in the presence of the acid digests the casein, gradually dissolving it, forming a straw-colored fluid containing peptones.  The peptonized milk has a peculiar odor and bitter taste.

Experiment 70. To show the action of rennet on milk.  Place milk in a test tube, add a drop or two of commercial rennet, and place the tube in a water-bath at about 100 degrees F. The milk becomes solid in a few minutes, forming a curd, and by and by the curd of casein contracts, and presses out a fluid,—­the whey.

  Experiment 71.  Repeat the experiment, but previously boil the rennet.  No
  such result is obtained as in the preceding experiment, because the
  rennet ferment is destroyed by heat.

Experiment 72. To show the effect of the pancreatic ferment (trypsin) upon albuminous matter.  Half fill three test tubes, A, B, C, with one-per-cent solution of sodium carbonate, and add 5 drops of liquor pancreaticus, or a few grains of Fairchild’s extract of pancreas, in each.  Boil B, and make C acid with dilute hydrochloric acid.  Place in each tube an equal amount of well-washed fibrin, plug the tubes with absorbent cotton, and place all in a water-bath at about 100 degrees F.

Experiment 73.  Examine from time to time the three test tubes in the preceding experiment.  At the end of one, two, or three hours, there is no change in B and C, while in A the fibrin is gradually being eroded, and finally disappears; but it does not swell up, and the solution at the same time becomes slightly turbid.  After three hours, still no change is observable in B and C.