It must be constantly borne in mind that anything but mother's milk is a foreign substance in the stomach of the child, and even to make a mixture agreeing in chemical composition with the natural food is not to insure success. The general principles must be applied in a special way to each individual case.

The first step in modification is the dilution of the protein. Few new-born infants can digest pure cow's milk. Some require more dilution than others. Where the digestive powers are unknown, it is wise to begin with a low strength and advance as rapidly as the condition of the child will permit. From ¼ to 1 per cent. protein is commonly recommended for the early weeks.

The second step is the adjustment of the fat. Individual infants differ in their tolerance of fat as of protein. But inasmuch as the chemical and physical differences are not as great in the case of fat, it is usually not necessary to reduce it below 2 per cent., and it should approximate the proportion in mother's milk as rapidly as possible.

When whole milk is diluted, if more than one volume of water be added, the percentage of fat is less than two, and for greater fat content, fat must be added in the form of cream. Since it is difficult to get the right percentage in this way, it is customary to select a milk with such a ratio between the protein and fat that diluting one to the desired strength dilutes the other also to the required degree. This is accomplished by taking the upper layers of milk which has been standing five hours or more for the cream to rise. The upper layers have much fat and little protein; the lower, much protein and little fat, and the adjustment is readily made by taking a certain number of ounces for dilution. In the laboratory, the exact fat and protein content can be determined by direct analysis.

The third step in modification is the adjustment of the milk sugar. This can usually be given of the full strength found in mother's milk (7 per cent.). Since lactose commercially prepared from cow's milk is of the same composition as that in mother's milk, it is only necessary to know the percentage already present in the mixture to be fed, and then add sufficient to make the desired strength.

Besides these three main steps in artificial feeding, we may have as a further consideration the selection of a diluent. Water is the first choice, but to aid in the modification of the character of the curd, cereal waters or gruels, lime water, proprietary foods, etc., are sometimes prescribed. Carefully prepared, and regarded mainly as diluents and not as substitutes for milk, the cereals and prepared foods are often useful aids. Lime water causes the mucin of the milk to swell, and tends to send the curd in soft condition into the intestines. It is also beneficial as a corrective of constipation.

What Dilution To Use

The degree of dilution varies with the age of the child. Advance is made from low strength to higher as rapidly as the child is able to digest stronger food. Since it is usually necessary to begin with a formula weaker than mother's milk, it is desirable that stronger formulae be given as soon as practicable to compensate for this early loss. This is particularly true as regards mineral constituents. On diluted cow's milk, the child gets less iron and phosphorus than in mother's milk, and while cow's milk is rich in calcium, it is doubtful whether this is as perfectly utilized as that in human milk.

The Quantity To Feed

The amount of milk taken by the breast-fed child is the usual guide as to quantity. (See Capacity of Baby's Stomach.) Both under-feeding and over-feeding are undesirable. The best criteria of proper feeding are the normal weekly gain in weight (averaging at least four ounces) and the general well-being of the child. A determination of the energy-value of the diet is desirable as a control. For the first year, a child requires on the average 100 calories per kilogram of body weight. Seventy calories per kilogram is minimum for existence, and does not ordinarily provide for storage in growth.

Laboratory Milk

With the development of the milk laboratory, it has become possible to obtain by prescription, pure milk of any given composition. Such milk is necessarily expensive, and since slight variations in the composition of formulae are not serious for the ordinary healthy child, the use of laboratory milk is largely limited to sick babies or those of rich parents.

Home Modification

For the average child, milk can be successfully modified at home. The chief requirements are fresh clean milk, absolute cleanliness in its treatment, and careful following of directions in making up the feedings.

Clean raw milk is preferable to any other kind. Therefore certified milk should be used if it can be afforded. If this is impossible, then the best bottled milk. If this is pasteurized, the pasteurization should be at a low temperature. Loose grocery milk should be avoided. When the milk comes into the house, the bottles should be carefully washed, and set in the cold until needed.

All apparatus used in preparing the baby's food should be kept by itself on a tray or table. Every vessel should be washed in soapsuds and then scalded in clear water before use.

If there is a special nursing refrigerator, it should be cleansed daily with soda-water.