When canned or condensed milk and cream are alone available or when we wish to imitate the methods of milk laboratories, the following formulae may be used (Author's method, with assistance of Prof. M. M. Ward well).

pf" - fp" x= p'f" - f'p"a fp'-f'p y= p'f" - fp"a w = a - (x + y)

In these equations, a is the number of c.c. (or other units; of the modified milk mixture, desired for a feeding or for a day.

x is the number of units of milk containing approximately the same proportionate ingredients as ordinary cow's milk.

y is the number of units of cream.

w is the number of units of water.

p, f, and 1, represent grams per hundred of protein, fat and lactose. Without primes they indicate the percentages in the desired mixture; with single primes, the same for milk; with seconds, the same for cream. To determine the amount of lactose to be added, we have the equation: z = la - (l' x + l"y)

If it is desired to be very accurate, all quantities may be estimated by weight (grams instead of c.c.) and, in this case, the equation for water becomes w = a - (x-f-y + z)

Note that in this, as all cases in which a correspondence between units of weight and of volume is assumed, there is no exact correspondence between the volumetric and gravimetric drachm, and ounce or between the pint and pound.

Human milk is persistently though faintly alkaline, while cow's milk is usually feebly acid. The former has a slightly higher specific gravity (1031.3 as against 1029.7 for cow's milk) and slightly more solids (13.33% as against 12.61). Human milk sucked from the breast is sterile or nearly so while it is practically impossible, with all precautions, to deliver cow's milk with less than 10,000 bacteria per c.c. Hygienic dairies frequently deliver milk containing 40,000 bacteria per c.c. and 100,000 is about the best result to be expected from commercial dairies. Even this number, coasisting mainly of comparatively harmless saprophytes, usually produces no trouble. 1,000,000 bacteria are often found in a c.c. of ordinary commercial milk.

Dairy milk should contain about 3.75% of fat, human milk 4.13%. The respective protein composition is 3.76% and 2%; lactose 4.42 and 7%; ash 0.68 and 0.2%. Good cream contains 3.5% protein, 25% of fat (20% is the more common commercial average) and 3.5% of lactose. These percentages may be used in the algebraic computation of artificial milk mixtures.