The structure of a long bone.—The long bones of the limbs are hollow except at the ends. Strength and lightness are thus secured by a device (the hollow cylinder), which has already (p. 72) been seen to be adopted by the supporting structures of plants, and is also copied by [Pg 229] human engineers. The cavity of the tube is filled by marrow, which supplies the bone with food. The hard, bony tube itself is partly composed of mineral matter and partly of animal matter. The mineral matter is left as a white, brittle framework when the animal matter is burnt away; while on the other hand the mineral part—which gives the bone its rigidity—may be dissolved out by immersing the bone in dilute hydrochloric acid, leaving the organic tissue as a soft flexible substance having the shape of the original bone. 45. HOW THE RABBIT DIGESTS ITS FOOD. 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 [Pg 230] 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.