L. Emmet Holt furnishes the following table of variations in specific gravity of human milk and their causes, as determined by a lactometer of his own device:

Human Milk (Holt)

"Milk presenting only moderate variations from the average - e. g., specific gravity 1,028, cream 4 per cent, or specific gravity 1.033, cream 10 per cent - can usually be modified by appropriate treatment. If, however, the specific gravity is from 1.018 to 1.024, and cream only 2 per cent to 3 per cent, it is hopeless." (Holt).

Estimation Of The Fat

To estimate the amount of fat or cream the sample of milk is allowed to stand in a cool place for twenty-four hours in a "creamometer." This is a simple glass tube twenty-five millimetres wide and twenty-five centimetres deep, which is graduated in hundredths from above downward. It is filled with milk to the zero level. The cream rises to the top, and the percentage of volume is read off.

This method is amplified by Marchand's process, in which a similar graduated tube is employed, but the cream is more completely separated by means of the addition of ether and a little liquor sodae. Ninety-five per cent alcohol is then put into a flask with the milk, and the whole is vigorously shaken, after which it is allowed to stand at 1300 to 1400 F. In half an hour the fat forms a distinct layer at the top of the graduated tube. The error by this process is said to be less than 0.3 per cent.

Dilution of milk by water is determined in the New York Health Board by use of a Swedish separating machine, the principle of which is as follows: Glass tubes are used having a diameter of about half an inch with a contracted neck graduated in percentages. A minute perforation at the neck admits the entrance of water. The tubes are first nearly filled with sulphuric acid and a measured quantity of milk is added. The tubes are corked with rubber, placed in a pan filled with water at 1500 F., and rotated in a centrifugal machine at a speed of 40,000 revolutions a minute. The temperature of the water is then reduced to 520 F., and the percentage of fat is read off from the narrow neck of the tubes. Water is found to have entered through the perforation, and by its intermediate specific gravity it lies between and separates the fat and sulphuric acid. The process requires half an hour's time, and twenty samples may be examined at once.

The richness of the milk in cream, or its dilution with water, may also be measured by its opacity. Five cubic centimetres of water are placed in a glass cell, called a "lactoscope " or "diaphanometer" (Dome), with parallel sides, through which a candle placed at a distance of a metre may be seen. More milk is then added, drop by drop, until the opacity of the cell contents obscures the light. This method is more tedious and less accurate than the previous one. Cream varies in specific gravity between 1.010 and 1.024.

A simple method of roughly determining the quantity of fat in human milk is given by Nias. The milk is placed in a test tube, made distinctly alkaline by a few drops of liquor potassae, boiled, and left for a few hours in a warm place. The fat, floating on the surface, is then estimated by graduations on the side of the test tube. It is of course necessary to use test tubes of uniform calibre in making comparative tests.

Estimation Of Lactose

The method of quantitative estimation of milk sugar is the same as that employed for glucose in the urine. The casein is first precipitated by acetic acid. The whey is filtered and tested with Fehling's copper solution or with a polariscope. Convenient tables are made to facilitate the calculation.

Estimation Of Albuminoids

In the method of Nias to determine the albuminoids the sample of milk is placed in a graduated test tube, as if to estimate the cream, and after some hours the supernatant fat is drawn off by a pipette. Acetic acid is next added until a strongly acid reaction develops. The milk is then boiled. All the albuminoids are precipitated, and after standing overnight the quantity may be read off on the side of the graduated tube.