It is obvious that the more nearly the substitute approaches to the character of the mother's milk, the greater will be the prospect of the attempt to rear the child upon it proving successful. There is no argument needed to prove that the milk of some animal more closely resembles the mother's milk, and is more likely to prove a useful substitute for it than any kind of farinaceous substance. The milk of all animals, however, differs in many important respects from human milk, and differs too very widely in different animals. Thus, the milk of the cow and that of the ewe contain nearly double the quantity of curd, and that of the goat more than twice the quantity of butter, and it is only in the milk of the ass that the solid constituents are arranged in the same order as in man. On this account, therefore, asses' milk is regarded, and with propriety, as the best substitute for the child's natural food. Unfortunately, however, expense is very frequently a bar to its employment, and compels the use of the less easily digested cows' milk. But though the cost may be a valid objection to the permanent employment of asses' milk, it is yet very desirable when a young infant cannot have the breast, that it should be supplied with asses' milk for the first four or five weeks, until the first dangers of the experiment of bringing it up by hand have been surmounted. The deficiency of asses' milk in butter may be corrected by the addition of about a twentieth part of cream, and its disposition to act on the bowels may be lessened by heating it to boiling point, not over the fire but in a vessel of hot water; and still more effectually by the addition to it of a fourth part of lime-water or of a teaspoonful of the solution of saccharated carbonate of lime to two ounces or four tablespoonfuls of the milk. When cows' milk is given, it must be borne in mind that it contains nearly twice as much curd, and about an eighth less sugar, than human milk. It is therefore necessary that it should be given in a diluted state and slightly sweetened. The dilution must vary according to the infant's age; at first the milk may be mixed with an equal quantity of water, but as the child grows older the proportion of water may be reduced to one-third. Mere dilution with water, however, leaves the proportion of curd unaltered, and it is precisely the curd which the infant is unable to digest. Instead, therefore, of diluting the milk simply with water, it is often better to add one part of whey to about two parts of milk, which, according to the child's age, may or may not be previously diluted.[8]