If the student is versed in chemistry, this lesson will serve merely as a review; if not, somewhat close attention must be given to facts which at first may seem uninteresting. Patience should be exercised, for, while all the information herein given does not, taken as a whole, bear directly upon the subjects of health and disease, yet with this knowledge it will be much less difficult to understand the principles which are applied later when we take up the chemistry of the body and the chemistry of food.

Chemistry is not, as popularly supposed, a science far removed from everyday life. Everyone has some knowledge of chemistry, but the chemist has observed things more minutely and therefore more accurately understands the composition of substances and the changes that are everywhere taking place. For illustration:

Relation of chemistry to food science.

A cook starts a fire in a stove. She knows that the fire must have "air" or it will not burn; that when the fire is first lighted, it "smokes" heavily, but as it burns more, it smokes less; further, that if the damper in the pipe is closed the "gas" will escape in to the room.

The chemist also knows this, but because he has compared his observations with similar events elsewhere, he is enabled to express his knowledge in the language of science. To the chemist, fire is the process of combustion - the union of the oxygen of the air with the carbon and hydrogen compounds of the wood or of the coal. The heat of the fire is generated by this chemical union. To the chemist, the smoke is a natural phenomenon occasioned by particles of carbon which fail to unite with the oxygen gas. The gas, which to the woman suggests suffocation if enough of it escapes into the room, to the chemist suggests a compound resulting from combination of the oxygen with the carbon.

Fire, gas, and smoke the result of chemical changes.