problem how energy is to be obtained from substances so unlike as those I have mentioned? We get light on this question by examining a natural food, one upon which almost all men live in the earlier part of their existence, the food of childhood, milk.
I pour a glass of milk into a basin and warm it, adding a few drops of acetic acid. You see it quickly undergoes a change. Masses of curd make their appearance and float in a fluid of a yellowish colour, familiarly known as the whey. We filter it. The curd you see is a soft friable matter. It consists of a body called casein, of very complex chemical composition. If we gave it to a chemist to analyse, he would tell us that it contained carbon, hydrogen, oxygen, and nitrogen. Note particularly that it contains nitrogen; it is, as we say, a nitrogenous substance, and it represents the first constituent of every diet, which must contain a nitrogenous or, as it is termed, a proteid substance. Proteid bodies include such substances as we find, along with other matters, in white of egg, in meat, in wheat, in oats, in beans, in grass, and in many other well-known articles of diet. The casein.
