An Introduction to Nature-study - E. Stenhouse

Fig. 162.—Experiment to show
that starch solution will not
pass through a thin membrane.
b, beaker; w, water;
p.p., parchment paper;
t.t., stem of thistle funnel;
s,starch solution in head
of thistle funnel.

1. A solution of starch will not pass through a thin membrane.—Rub up with water as much starch as will lie on a shilling, so as to form a thin “cream,” and then pour on it about a cupful of boiling water. The starch swells up and largely dissolves in the water. Add a few drops of the starch solution to about half a pint of water, stir, and test it by adding a little iodine solution (p.2, [footnote]). A beautiful blue colour is obtained, showing that the test is a very delicate one. Now take a thistle funnel and with a file cut through the stem about six inches below the head. Wet a piece of parchment paper or thin bladder (having previously held it up to the light to be sure there are no holes in it), and tie it tightly across the mouth of the funnel. Fill the head and about an inch of the stem with the starch solution. This can easily be done by means of a “canula,” such as is used for filling fountain pens. Now put the funnel into a beaker of water, in the manner shown in [Fig. 162], and put the arrangement aside for a few hours. After that time add iodine solution to the water in the beaker. No blue colour is formed, showing that no starch has passed through the membrane.

2. A solution of sugar will pass through a thin membrane.—A delicate test for certain varieties of sugar (not, however, table sugar) is a liquid known as Fehling’s solution.[10] Place a particle of honey in a test tube with a teaspoonful of Fehling’s solution, and put the test tube into a vessel of boiling water. Notice that in a short time the blue colour of the solution disappears and the liquid becomes red and turbid.

Now repeat Experiment 45, 1, but instead of starch solution use honey dissolved in water. To show that some of the honey-sugar has passed through the membrane, take about a teaspoonful of the water in the beaker, and put it in a test tube with twice as much Fehling’s solution. Heat as before, and notice the red turbidity. If table sugar is used it may be recognised by the sweet taste of the water after the experiment.

3. The action of saliva on starch.—(a) Chew a piece of india-rubber to induce a free flow of saliva, and collect the liquid. In one test tube put half a teaspoonful of starch solution; in a second tube put a mixture of equal quantities of starch solution and saliva; in a third put saliva alone. Keep the tubes at blood heat for twenty minutes. Then add a little water to each tube and divide its contents into two parts. Test one part of each for starch with iodine solution, and the other part for sugar with Fehling’s solution. The first tube contains only starch. The second now contains no starch, but shows the presence of sugar. The third contains neither starch nor sugar. Evidently the saliva has changed the starch of the second tube into sugar.

(b) Repeat the experiment, but keep the mixture of starch and saliva in a cold place. No sugar is produced.

The necessity for food.—It is common knowledge that a rabbit, like every other animal, must have a regular supply of food if it is to continue healthy, and that it would soon die outright if food were withheld. The reason for this is that the living substance of the animal’s body is incessantly wasting away. The rabbit cannot move a muscle except at the expense of the living substance of the muscle, and the more active is its life, the more rapidly does its body waste. It is to counteract this continual waste by continual formation of new living substance that food is taken.