Although, as has just been pointed out, moisture is required for the growth of some micro-organisms, both moisture and warmth are necessary for the growth of most of the organisms that cause molding, putrefaction, and fermentation. It is definitely known, also, that in winter or in cold climates food can be kept for long periods of time without any apparent change; in fact, the lower the temperature the less likely are foods to spoil, although freezing renders many of them unfit for use. These facts are what led up to the scientific truth that keeping foods dry and at a low temperature is an effective and convenient method of preventing them from spoiling and to the invention of the refrigerator and other devices and methods for the cold storage of foods.

THE REFRIGERATOR.--For home use, the refrigerator offers the most convenient means of keeping foods in good condition. As is well known, it is a device that, by means of air cooled by the melting of ice or in some other manner, keeps food at a temperature near the freezing point. All refrigerators are constructed in a similar manner, having two or more layers of wood between which is placed an insulating material, such as cork, asbestos, or mineral wool. The food compartments are lined with tile, zinc, or other rust-proof material, and the ice compartment is usually lined with rust-proof metal, so as to be water-tight and unbreakable. Any refrigerator may be made to serve the purpose of preserving food effectively if it is well constructed, the ice chamber kept as full of ice as possible, and the housewife knows how to arrange the foods in the food chambers to the best advantage.

The construction and use of refrigerators are based on the well-known scientific fact that air expands and rises when it becomes warm. This can be proved by testing the air near the ceiling of a room, for no matter how warm it is near the floor it will always be warmer above. The same thing occurs in a refrigerator. As air comes in contact with the ice, it is cooled and falls, and the warm air is forced up. Thus the air is kept in constant motion, or circulation.

Many refrigerators are built with the ice compartment on one side. In such refrigerators, there is usually a small food compartment directly under the ice chamber, and this is the coldest place in the refrigerator. Here should be stored the foods that need special care or that absorb odors and flavors readily, such as milk, butter, cream, meat, etc., because at this place the air is the purest. The foods that give off odors strong enough to taint others should be kept on the upper shelves of the refrigerator, through which the current of air passes last before being freed from odors by passing over the ice.

Another type of refrigerator is one in which the ice chamber, or compartment, extends across the entire top. This type is so built as to produce on each side a current of air that passes down from the ice at the center and back up to the ice near the outside walls, as shown by the arrows. A different arrangement is required for the food in this kind of refrigerator, those which give off odors and flavors being placed in the bottom compartment, or farthest from the ice, and those which take up odors and flavors, on the top shelf, or nearest the ice.

CARE OF FOOD IN REFRIGERATOR.--The proper placing of foods in a refrigerator is extremely important, but certain precautions should be taken with regard to the food itself. Cooked foods should never be placed in the refrigerator without first allowing them to cool, for the steam given off when a dish of hot food comes in contact with the cold air makes the refrigerator damp and causes an undue waste of ice by warming the air. All dishes containing food should be wiped dry and carefully covered before they are placed in the refrigerator, so as to keep unnecessary moisture out of it. As butter and milk are likely to become contaminated with odors given off by other foods, they should be properly protected if there is not a separate compartment in which to keep them. The milk bottles should always be closed and the butter carefully wrapped or put in a covered receptacle. Onions, cabbage, and other foods with strong odors, when placed in the refrigerator, should be kept in tightly closed jars or dishes, so that the odors will not escape. Before fresh fruits and perishable vegetables--that is, vegetables that decay easily--are put into the refrigerator, they should be carefully looked over and all decayed portions removed from them. No food should be placed in the ice chamber, because this will cause the ice to melt unnecessarily.

CARE OF THE REFRIGERATOR.--It is essential that all parts of the refrigerator be kept scrupulously clean and as dry as possible. To accomplish this, nothing should be allowed to spoil in it, and anything spilled in the refrigerator should be cleaned out immediately. The foods that are left over should be carefully inspected every day, and anything not likely to be used within a day or so should be disposed of. At least once a week the food should be removed from all compartments, the racks taken out, the drain pipe disconnected, and each part thoroughly washed, rinsed with boiling water, and dried. The inside of the refrigerator should likewise be washed, rinsed, and wiped dry, after which the drain pipe should be connected, the shelves put back in place, and the food replaced.

The ice chamber of the refrigerator should also be cleaned frequently, the best time to do this being when the ice has melted enough to be lifted out conveniently. To prevent the ice from melting rapidly when it is out of the refrigerator, it may be wrapped in paper or a piece of old blanket, but this covering must be removed when the ice is replaced in the chamber, in order to allow the ice to melt in the refrigerator. Otherwise, it would be impossible to chill the refrigerator properly, the temperature remaining the same as that outside, for it is as the ice gradually melts that the air in the refrigerator becomes cool. Of course, every effort should be made to keep the ice from wasting. Therefore, while the refrigerator should be kept in a convenient place, it should not be exposed to too great heat; also, the doors should be kept tightly closed, and, as has already been explained, hot foods should not be put in until they are sufficiently cooled. Attention must be given to the care of the refrigerator, for only when it is clean and dry can the growth of bacteria that attack foods be prevented.