--Flint's _Human Physiology_, page 293.

Experiment 63. Stretch a piece of moist bladder across a glass
tube,--a common lamp-chimney will do. Into this put a strong saline
solution. Now suspend the tube in a wide mouthed vessel of water. After
a short time it will be found that a part of the salt solution has
passed through into the water, while a larger amount of water has passed
into the tube and raised the height of the liquid within it.

161. The Quantity of Food as Affected by Age. The quantity of food
required to keep the body in proper condition is modified to a great
extent by circumstances. Age, occupation, place of residence, climate, and
season, as well as individual conditions of health and disease, are always
important factors in the problem. In youth the body is not only growing,
but the tissue changes are active. The restless energy and necessary
growth at this time of life cannot be maintained without an abundance of
wholesome food. This food supply for young people should be ample enough
to answer the demands of their keen appetite and vigorous digestion.

In adult life, when the processes of digestion and assimilation are
active, the amount of food may without harm, be in excess of the actual
needs of the body. This is true, however, only so long as active muscular
exercise is taken.

In advanced life the tissue changes are slow, digestion is less active,
and the ability to assimilate food is greatly diminished. Growth has
ceased, the energy which induced activity is gone, and the proteids are no
longer required to build up worn-out tissues. Hence, as old age
approaches, the quantity of nitrogenous foods should be steadily
diminished.

Experiment 64. Obtain a sheep's bladder and pour into it a heavy
solution of sugar or some colored simple elixir, found at any drug
store. Tie the bladder carefully and place it in a vessel containing
water. After a while it will be found that an interchange has occurred,
water having passed into the bladder and the water outside having become
sweet.

Experiment 65. Make a hole about as big as a five-cent piece in the
large end of an egg. That is, break the shell carefully and snip the
outer shell membrane, thus opening the space between the outer and inner
membranes. Now put the egg into a glass of water, keeping it in an
upright position by resting on a napkin-ring. There is only the inner
shell membrane between the liquid whit