[Sidenote: Classes vs. groups of related compounds]
In the food tables and analyses commonly published, the above terms are used with very little explanation, and read by the average person with meager comprehension. When one reads that a food is composed of glucose, citric acid, or globulin, he is likely to become confused, not being able to understand how a food at one time can be said to be composed of carbohydrates, proteids, and fats, and at another time to be composed of other substances. The explanation is that the first classification does not refer to definite chemical substances, but to groups of related compounds having properties in common.
[Sidenote: The different methods of analyzing food]
There is still another way of giving the chemical composition of a food, namely, to specify the chemical elements that it contains. It will be remembered that the relation between chemical elements and chemical compounds was explained in the first lesson. As an example, I will take the analysis of milk. We will first say that milk contains a certain percentage of protein, carbohydrates, and fat. We might then say that the proteid of milk is part casein and part albumin, and that the albumin contains certain percentages of oxygen, sulfur, etc.; also that the chief carbohydrate in milk is milk-sugar, which in turn is composed of carbon, hydrogen, and oxygen. Or, we could consider the milk as a whole, without dividing it into groups, and give the per cent of each chemical element in the milk. Thus, the carbon of the proteid, milk-sugar, and fat would be all considered together, and show a certain per cent of carbon in the milk as a whole.
The word _carbohydrate_ means _carbon combined with water_; that is, the element carbon is combined with hydrogen and oxygen, which exist in the carbohydrate compound in the same proportion as they exist in water.
The carbohydrates are closely related chemically to the aldehydes and the alcohols, so far as their composition is concerned (See "Aldehydes and Ethers," Lesson III, p. 93), but this does not imply that they have the same physiological effect in the animal body.
The carbohydrates are divided by the chemist into three classes known as