"Ah! I wonder that that force can group the atoms that we breathe, or that we assimilate by nutrition and form this charming being! Think of that young girl the day she was born, and follow in thought the gradual development of that little body through the years of her awkward age to the first graces of youth and the charms of womanhood. How is human organism nourished, developed, and composed? You know,—by respiration and nutrition.
"The air supplies three quarters of our nourishment by respiration. The oxygen in the air maintains the fire of life, and the body is comparable to a flame, constantly renewed by the principles of combustion. The lack of oxygen extinguishes life as it extinguishes a lamp. By respiration the black venous blood is transformed into red arterial blood and regenerated. The lungs are a fine tissue pierced with from forty to fifty millions of little holes, which are just too small for the blood to filter through, and just large enough for the air to penetrate them. A perpetual interchange of gas takes place between the air and the blood, the first furnishing the second with oxygen, the second eliminating carbonic acid. On the one hand the atmospheric oxygen burns carbon in the lung; on the other the lung exhales carbonic acid, nitrogen, and water in the form of vapor. In the daytime, plants breathe by an opposite process,—they absorb carbonic acid and exhale oxygen; by this difference maintaining one part of the general equilibrium of terrestrial life.
"Of what is the human body composed? An average adult man weighs 70 kilograms. Of this amount there are nearly 52 kilograms of water in the blood and flesh. Analyze the substance of our body, you will find albumen, fibrine, caseine, and gelatine; that is, organic substances composed originally of the four essential gases,—oxygen, nitrogen, hydrogen, and carbonic acid. You will also find substances with no nitrogen, such as gum, sugar, starch, and fat. These matters likewise pass through our organism; their carbon and hydrogen are consumed by the oxygen breathed in during respiration, and then exhaled under the form of carbonic acid and water.
"You are not unaware that water is a combination of two gases, oxygen and hydrogen; the air is a mixture of two gases, oxygen and nitrogen, to which are added in lesser proportions water in the form of vapor, which, however, is but condensed oxygen, etc.
"Thus our body is composed only of transformed gases."
*****
"But," interrupted my companion, "we do not live solely upon the air; at certain hours, indicated by our stomachs, it is very necessary to add some supplies which are not without a value of their own,—such as a pheasant's wing, a filet de sole, a glass of Château Laffitte or champagne, or, as your taste may prefer, asparagus, grapes, peaches...."
"Yes, that all passes through our organism and renews its tissues,—pretty rapidly too; for in a few months (not in seven years, as was formerly thought) our body is entirely renewed. To return to that lovely being who posed before us just now. None of that flesh which we admired existed three or four months ago; those shoulders, that face, those eyes, that mouth, those arms, that hair, and, even to the very nails, all that organism, is but a current of molecules, a ceaselessly renewed flame, a river which we may look at all our lives, but never see the same water again. Now, all that is but assimilated gas, condensed and modified, and more than anything else, it is air. These bones themselves, so solid now, were formed and hardened gradually. Do not forget that our whole body is composed of invisible molecules which do not touch each other, and which are continually renewed.
"Finally, our table is spread with vegetables and fruits; if we are vegetarians we absorb substances almost entirely drawn from the air. This peach is air and water; this pear, this grape, this almond are also made of air and water, a few gaseous elements drawn to them by the sap, by solar heat, by the rain. Asparagus or salad, peas or beans, lettuce or chicory, all these live in the air and on the air; what the earth furnishes, what the sap seeks out, are also gases, and the very same nitrogen, oxygen, hydrogen, carbon, etc.
"If it is a question of beefsteak, chicken, or some other 'meat,' the difference is not very great. Sheep and oxen feed upon grass. If we relish a partridge cooked with cauliflower, a roasted quail, a truffled turkey, or a stewed hare, all these substances, apparently so different, are only transformed vegetable matter, which itself is but a grouping of molecules taken from the gases of which we have just been speaking,—air, water, elements, molecules, and atoms almost imponderable of themselves, and moreover absolutely invisible to the naked eye.