taining a double scale, on the lower part of which the specific gravity is read, while the temperature is read from the upper part.

Another form of instrument is termed the New York Board of Health lactometer, which is not graduated to read the specific gravity directly, but has an arbitrary scale divided into 120 equal parts, the zero being equal to the specific gravity of water, while 100 corresponds to a specific gravity of 1.029. Deghuée * has devised a special form requiring only 4 ounces of milk. To convert readings on the New York Board of Health scale to Quevenne degrees they must be multiplied by .29.

QUEVENNE LACTOMETER DEGREES CORRESPONDING TO NEW YORK BOARD OF HEALTH LACTOMETER DEGREES.

If extreme accuracy is desired, the Westphal balance or the pycnometer should be used for the determination of specific gravity. For ordinary cases, however, the lactometer, if carefully made, is sufficiently accurate.

With any other form of lactometer than the Quevenne, a separate thermometer is necessary in order to determine the temperature, the common practice being to standardize all such instruments at 60° F. (15.6° C.).

Readings at temperatures other than 60° may be corrected to that temperature by the aid of the table on page 133.

-

DETERMINATION OF TOTAL SOLIDS.-Dish Method. For purposes of milk analysis, platinum dishes are by far the most desirable. These, if made for the purpose, should be of the shape shown in Fig. 51, measur

* Jour. Ind. Eng. Chem., 3, 1911, p. 405.

FOR CORRECTING THE SPECIFIC GRAVITY OF MILK ACCORDING TO TEMPERATURE (BY DR. PAUL VIETH).

Degrees of

Lactom

eter.

20.....

21.

22

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

33

34.

35

20.. 21.

22...

23.

24.

25

26.

27.

28.

29.

30.

31

32.

33

34

35

45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

19.0 19.0 19.119.1 19.2 19.2 19.3 19.4 19.4 19.5 19.6 19.7 19.8 19.9 19.9 19.9 20.0 20.0 20.120.220.220.3 20.3 20.4 20.5 20.6 20.7 20.8 20.9 20.9 20.9 21.0 21.0 21.121.2 21.2 21.3 21.3 21.4 21.5 21.6 21.7 21.8 21.9 21.9 21.9 22.0 22.0 22.122.222.222.3 22.3 22.4 22.5 22.6 22.7 22.8 22.8 22.9 22.922.9 23.023.123.223.223-323-323-423-523-623.623-723-823-9 23.823.9 24.0 24.0 24.124-124-224-324-424-524-624-624-724-824-9 24-824-924-925.025.1 25.125.225.225-325-425-525-625-7 25.8 25.9 25.8 25.9 25.9 26.0 26.1 26.1 26.2 26.2 26.3 26.4 26.5 26.6 26.7 26.8 26.9 26.7 26.8 26.8 26.9 27.0 27.0 27.1 27.227-327-427-527-627-727-8 27.9 27.727.8 27.8 27.9 28.0 28.0 28.1 28.2 28.3 28.4 28.5 28.6 28.7 28.8 28.9 28.628.7 28.7 28.8 28.9 29.0 29.129.129.2 29.3 29.4 29.6 29.7 29.8 29.9 29.5 29.6 29.629.7 29.8 29.9 30.0 30.130.230.3 30.4 30.5 30.630.8 30.9 30.4 30.5 30.5 30.630.7 30.9 31.031.131.231.3 31.431-531-631-731-9 31-331-431.4 31.5 31.631.831.9 32.0 32.1 32-332.4 32.5 32.632-732.9 32.232-332-334-4 32-532-732-933-033-133-233-333-533-633-733-9

33-033-133-233-433-533-633-833-934-034-234-334-534-634-734-9

61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 20.1 20.220.220.3 20.4 20.5 20.6 20.7 20.9 21.0 21.1 21.2 21.3 21.5 21.6 21.121.2 21.3 21.4 21.5 21.6 21.7 21.8 22.022.1 22.2 22.3 22.4 22.5 22.6 22.122.222.3 22.4 22.5 22.622.7 22.823.023.1 23.223-323-423-523-7 23-123-223-323-423-523.623.7 23.8 24.0 24.1 24.2 24-324-424-624-7 24.1 24-224-324-424-5 24.624-724-925.025.125.225-325-525.6 25.7 25-125-225-325-425-5 25.625.725-9 26.0 26.1 26.2 26.4 26.5 26.626.8 26.1 26.2 26.3 26.5 26.6 26.7 26.8 27.0 27.1 27.2 27-327-427-527-727.8 27-127-327-427-527-627-727.8 28.0 28.1 28.2 28.3 28.4 28.6 28.7 28.9 28.1 28.3 28.4 28.5 28.6 28.7 28.8 29.0 29.1 29.229.4 29.5 29.729.8 29.9 29.129.3 29.4 29.5 29.6 29.8 29.9 30.130.2 30.3 30.4 30.5 30.7 30.9 31.0 30.1 30.3 30.4 30.5 30.7 30.8 30.9 31.131.2 31.3 31.5 31.6 31.8 31.9 32.1 31.231.3 31.4 31.5 31-731-7 31.8 32.0 32.2 32.4 32.5 32.6 32.8 33.033.1 32.232.332.5 32.6 32.7 32.9 33-033-233-333-433-633-733-934-034.2 33-233-333-533-633-833-934-034-234-3 34-534-634-734-935-135.2 34-234-334-534-634-8 34-935-035-235-335-5 35.6 35.8 36.0 36.1 36.3 35-235-335-535-635-835-9 36.1 36.2 36.4 36.5 36.7 36.8 37.037-237-3

ing about 2 inches in diameter at the top, and 2 inches in diameter at the bottom, having carefully rounded rather than square edges, and being inch deep. The bottom is not perfectly flat, but slightly crowned outward. Such a dish will hold about 35 cc.

For purposes of economy it is best to have these dishes spun out with a thick bottom, but with thin sides, not so thin, however, as to be too readily bent.

If platinum dishes cannot be afforded, dishes of porcelain, glass, aluminum, nickel, or even tin may be used, but in all cases should be as thin as practicable.

About 5 cc. of the thoroughly mixed sample of milk are carefully

transferred by means of a pipette to a tared dish on the scale-pan, and its weight accurately determined. The dish with its contents is then transferred to a water-bath, being placed over an opening preferably but little smaller than the diameter of the bottom of the dish, so that as large a surface as possible is in contact with the live steam of the bath. Here it is kept for at least two hours, after which the dish is wiped dry while still hot, transferred to a desiccator, cooled, and weighed.*

Babcock Asbestos Method.†-Provide a hollow cylinder of perforated sheet metal, 60 mm. long and 20 mm. in diameter, closed 5 mm. from one end by a disk of the same material. The perforations should be about 0.7 mm. in diameter and about 0.7 mm. apart. Fill loosely with from 1.5 to 2.5 grams of freshly ignited, woolly asbestos, free from fine and brittle material, cool in a desiccator, and weigh. Introduce a weighed quantity of milk (between 3 and 5 grams), and dry in a wateroven to constant weight, which is usually reached after four hours' heating.

DETERMINATION OF ASH.-The platinum dish containing the milk residue, obtained in the determination of total solids by the dish method described above, is next placed upon a suitable support above a Bunsen flame (a platinum triangle or a ring stand is convenient for this), and the residue is ignited at a dull-red heat to a perfectly white ash, after which it is cooled and weighed.

DETERMINATION OF FAT.-Babcock Asbestos Method.-Extract the residue from the determination of water by the Babcock asbestos method with anhydrous ether in a continuous extraction apparatus, until all the fat is removed, which usually requires two hours. Evaporate the ether, dry the fat in the extraction flask at the temperature of boiling water, and weigh. The fat may also be determined by difference, drying the extracted cylinders at the temperature of boiling water.

* It is a common practice to transfer the milk residue, after a preliminary drying on the water-bath, to an air-oven, kept at a temperature of from 100° to 105°, where it is dried to a constant weight; but after an experience in analyzing over 30,000 samples of milk, the author is prepared to state that in his opinion the results obtained by the above method of procedure, using the water-bath alone, are more satisfactory. It is impossible to keep a milk residue at a temperature above 100° for any length of time without its undergoing decomposition, especially as to its sugar content, as is shown by the darkening in color. A milk residue should be nearly pure white, a brownish color showing incipient decomposition. Hence, by continued heating, especially at the temperature of 105°, the residue would continue to lose weight almost indefinitely. If it is thought best to give a final drying in the air-oven, the time should be short and the temperature employed should not in any case exceed 100°.

† U. S. Dept. of Agric., Bur. of Chem., Bul. 38, p. 100.

The Adams Method. For this method a strip of fat-free filter-paper about 2 inches wide and 22 inches long is rolled into a coil and held in place by a wire as shown in Fig. 44. Schleicher and Schüll furnish fatfree strips especially for this work, but it is very easy to prepare the strips and extract them with the Soxhlet apparatus.

About 5 grams of milk are run into a beaker with a pipette, and the weight of the beaker and milk are determined. The coil is then introduced into the beaker, holding it by the wire in such a manner that as much as possible of the milk is absorbed by the paper. It is often possible to take up almost the last drop of the milk. By then weighing the beaker, the amount of milk absorbed by the coil is determined by difference, and the paper coil is hung up and dried, first in the air and then in the oven at a temperature not exceeding 100°. Another method of charging the paper coil consists in suspending it by the wire and gradually delivering upon it 5 cc. of the milk from a pipette, the density of the milk being known.

FIG. 44.- The Adams Milkfat Coil.

The coil containing the dried residue is then transferred to the Soxhlet extraction apparatus (see p. 53) and subjected to continuous extraction with anhydrous ether for at least two hours, the receiving-flask being first accurately weighed. The tared flask with its contents is freed from all remaining ether, first on the water-bath and finally in the air-oven. It is then cooled and weighed, the increase in weight representing the fat in the amount of milk absorbed by the coil. If there is any doubt about all the fat having been extracted at first, the process of extraction may be continued till there is no longer a gain in weight of the flask. Experience soon shows the length of time necessary for the complete extraction, which of course depends on the degree of heat employed, and the frequency with which the extracting-tube overflows. Two hours is ample for most cases, in which the conditions are such that the ether siphons over from the extraction-tube ten times per hour.

FAT METHODS BASED ON CENTRIFUGAL SEPARATION.-These methods are the most practicable for commercial work and for use by the public analyst, since they are much more rapid, and, if carefully carried out, practically as accurate as the Adams method. They all depend upon the use of a centrifuge usually having hinged pockets in which are carried graduated bottles, into each of which a measured

quantity of milk is introduced. The milk is then subjected to the action of a suitable reagent, which dissolves the casein and liberates the fat in a pure state, after which, by whirling at a high speed, the pockets are thrown out horizontally and the milk fat driven into the neck of each bottle, where the amount is directly read.

The Babcock Test, although devised originally for the use of creameries and dairymen, is now extensively employed for fat determination in the laboratory. Leach found that the results by the Babcock test and the Adams method, obtained from time to time during ten years, agreed within narrow limits. The following figures show the results of such comparative determinations made in duplicate on three samples of milk, viz., a pure whole milk, (1) and (2); a watered milk, (3) and (4), and a milk centrifugally skimmed, (5) and (6).

COMPARATIVE FAT DETERMINATION BY ADAMS-SOXHLET AND BY BABCOCK PROCESSES.

Equally satisfactory results were obtained by Winton, using the Babcock asbestos method for comparison.

The Centrifuge.-Various styles of centrifuge, carrying from 2 to 40 bottles, are in use for this process.

Two forms of hand machine are shown in Fig. 45, one (D), for two bottles, arranged to screw on the edge of a table, the other for twelve bottles inclosed in a cast-iron case.

The number of revolutions of the revolving frame for each turn of the crank and the number of turns per minute necessary to secure the requisite number of revolutions of the frame should be determined once for all for each machine and the latter adhered to in making all

tests.