Licence: Public Domain Mark
Credit: The choice of food / by Henry Simpson. Source: Wellcome Collection.
Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.
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![Food has been defined as a substance which, when introduced into the body, suppUes material which renews some structure or maintains some vital process.* This definition will include some things which none of us ordinarily look upon as food—namely, the water we drink and the air we breathe—but which are of such importance that life with- out them would be impossible. Besides these there are certain mineral matters which form an essential part of our food. It includes moreover things with which we are familiar, such as meat, fish, butter, milk, bread, oatmeal, rice, potatoes, &c. Before we can clearly understand how these things can serve the purposes which we have spoken of as served by food, we must shortly tell you the composition of our bodies. Most of you know what is ]neant by a gas, and are aware that the air we breathe is a mixture of two gases, oxygen and nitrogen. You know, very likely, that charcoal, when it is pure, is called carbon. The purest form of carbon is the diamond; and it is a wonderful thing that a piece of black charcoal and the sparkling diamond can be made up of the same thing. Now carbon, oxygen, and nitrogen are found in our bodies, and hydrogen hkewise. Hydrogen is a gas which we do not find existing in a free state like carbon, oxygen, and nitrogen, but in combination with some other sub- stance. That water is such a combination of hydrogen and oxygen is easily shown by a chemical experiment. Certain proportions of these gases are mixed in a jar, and an electric spark passed through, when an explosion takes place, and instead of being merely mixed they have united in what is called chemical combinai tion, and formed a substance (water) quite unlike themselves. You can see the same thing if you hold a plate for a short time over the flame of a spirit lamp. While the plate is cool you have a dew deposited on it, from the union of the hydrogen of the spirit with the oxygen of the air. This, however, has taken place quietly, because it is gradual, not sudden. These four bodies—carbon, hydrogen, oxygen, and nitrogen- make up the greater portion of the body, and they are called' elements, because they have never been divided into bodies simpler than themselves. There are others of the elementary bodies which' I may have to mention, some of which are of great importance, but which occur in smaller quantities than those I have named, such as sulphur, phosphorus, chlorine, sodium, potassium, cal- *Dr. Edward Smith.](https://iiif.wellcomecollection.org/image/b21450456_0006.jp2/full/800%2C/0/default.jpg)
