+315. Reading the test.+--In a cream-test bottle the neck is so much wider that there is a much larger meniscus. In order to obtain an accurate result, the meniscus should be removed. This is done by carefully adding a substance called glymol, which is a mineral oil colored red. Usually about one-quarter of an inch of glymol is added to the fat column. This should not mix with the fat. The bottles should be placed in a hot water bath 135° to 140° F. for four minutes before reading. The temperature at reading should be 135° to 140° F. The reading is then taken from the bottom of the fat column to the line between this and the glymol. The bottle is graduated for 18 grams of material, but as only a part of 18 grams of cheese was used for the test, the reading should be multiplied by the part of 18 grams used. For example, suppose 6 grams of cheese were used and the test read 12 per cent fat. Since 6 is one third of 18, the actual percentage of fat is 3 times 12, or 36 per cent. +316. The Hart[134] casein test+ was devised to determine the percentage of casein in milk. A special test bottle and centrifuge are necessary. The method of making the test is as follows: Place 2 c.c. of chloroform in the casein test tube, add 20 c.c. of a 0.25 of 1 per cent solution of acetic acid at a temperature of 65° to 75° F. This solution of acetic acid is made by diluting 10 c.c. of glacial acetic acid with 100 c.c. of water, then dilute 25 c.c. of this solution to 1000 c.c. with water; 5 c.c. of milk at a temperature of 65° to 75° F. is then run into the bottle. The bottle is then covered with the thumb and inverted and the mixture shaken vigorously for exactly twenty seconds. It is then centrifuged within twenty minutes at a speed of 2000 revolutions a minute. The bottle should stand ten minutes before reading the percentage of casein. There are other tests for casein but they are very complicated. +317. Solids in the milk.+--Because not only the fat but all the solids are utilized in cheese-making, it is important to know the amount of the solids in the milk. This is ascertained by determining the specific gravity of the milk and knowing the fat-content; the solids not fat can then be calculated. +318. The lactometer.+--The specific gravity of liquids is measured by an instrument called a hydrometer. Its use is based on the fact that when a solid body floats in a liquid, it displaces a volume of liquid equal in weight to its own. Hydrometers are in many cases so made that the specific gravity can be read at the point where the scale is even with the upper surface of the liquid. A hydrometer that is especially adapted to milk is called a lactometer. There are two lactometers in common use, the Quevenne and the Board of Health. [Illustration: FIG. 71.--A Quevenne lactometer.]