The clearing away of the breakfast introduces to the housekeeper two important problems: - (i) the preservation of the remaining food from decay; (2) the proper cleaning of the articles used during the meal and its preparation.

Decay is caused by minute vegetable organisms known as moulds and bacteria. Both are present in the air either as the plants themselves or as their spores, the reproductive cells, ready to grow whenever they fall upon suitable soil. When these grow upon animal or vegetable substances, a variety of new compounds are formed, many of them taking oxygen from the air, so that finally the carbon becomes carbon dioxide, the hydrogen is oxydized to form water, and the other elements in their turn also become oxides, so that the decaying substance is utterly destroyed and new substances made in its place. When organic substances are protected from the action of these living plants, decay will not ensue.

The old idea was that oxygen caused decay, but many experiments disprove this. Oxygen alone does not produce this result, but oxygen with "germs" will do so. These "germs" develop much more slowly in the cold, so that food is placed in the refrigerator or in a cool place and away from the dust.

The problems introduced by these living plants, their life history and their work, as well as the methods of prevention and care against their ravages, belong rather to household bacteriology than to chemistry. We are ready therefore to pass on to our next problem, that of cleaning.

Decay Not Caused by Oxygen Alone