While the sample is heating it should be shaken from time to time, as this breaks up the blanket of casein on the surface and hastens the escape of moisture. As soon as the casein has lost its snow-white color, the cup should be removed from the flame. When the moisture has all been driven from the sample, a slightly pungent odor may be noticed. This may also be used as a guide to tell when the sample has been heated enough. The foam begins to subside at this point. Often one or two small pieces of casein are slow to give up their moisture. This is indicated by the snow-white color of the pieces. Evaporation can be hastened by shaking the sample with a rotary motion and thoroughly mixing these pieces with the hot liquid. If this is not done, one might have to heat the sample so long that some of the fat which had already given up its moisture would volatilize.

After all the moisture is driven off, the sample is allowed to cool to room temperature. While cooling, the cup should be covered with something (a sheet of paper will do) to prevent the sample taking up moisture from the atmosphere. After cooling, the cup is placed on the scales. The sample is lighter than before heating, because it has lost its moisture. The bar of the scales will therefore remain down. The weights are then reversed until the scales just balance.

Each notch that the larger weight is reversed has a value of 1 per cent (reading on the upper scale), and each notch that the smaller weight is reversed has a value of .1 per cent. If, for example, after heating, the scales just balance when the larger weight rests on 15 (upper scale) and the smaller weight rests on .2, it would mean that the sample contained 15.2 per cent moisture.

Test for salt in butter (Ross).

Weigh out accurately, from a well-mixed sample, 10 grams of butter. Add to the 10 grams of butter 100 cc. of hot water, and thoroughly mix the butter with the water. Then cool to harden the fat, and pour off into a clean dish the 100 cc. of water. Repeat this operation until 300 cc. of water has been used. Thoroughly mix the 300 cc. of water, and measure out 17.5 cc. into a glass beaker or white cup, and add five or six drops of potassium chromate. This will turn the solution a lemon-yellow color. Run in from a burette an n/10 normal solution of silver nitrate. Thoroughly mix the solution as the silver nitrate is added. When the solution turns to an orange-yellow color, enough silver nitrate has been added to neutralize all of the salt. The number of cc. of silver nitrate solution added equals the per cent of salt in the butter. For example, if it requires 2 cc. of silver nitrate, there is 2 per cent of salt in the butter. If more or less than 10 grams of butter are used and more or less than 17.5 cc. of the solution are used for the test, the burette will not give readings directly in terms of per cent. Care should be taken not to run in too much silver nitrate. If too much silver nitrate is used, the color will be a dull brick-red, and incorrect results will be obtained. An — normal solution of silver nitrate, which is accurate enough for the purpose, may be made by dissolving 17.5 grams of silver nitrate in 200 cc. of water and then making the solution to 1000 cc. or 1 liter.

Test for salt in cheese (Ross).

Burn to a gray ash in a porcelain dish 5 grams of the cheese. Care should be taken to keep the contents in the center of the dish. If this is done, it will make it easier to reduce the cheese to an ash.

Cool and dissolve the ash in 20 cc. of pure, clean water. Transfer the 20 cc. of the ash solution to a glass beaker or a white cup. Add five or six drops of a water solution of potassium chromate. This will turn the solution a lemon-yellow color. Run in from a burette an — normal solution of silver nitrate. Thoroughly mix the solution as the silver nitrate is added. When the color of the solution turns to an orange-yellow, enough silver nitrate has been added to neutralize all the salt. Then multiply the number of cc. of silver nitrate used by .00585. Divide this result by 5, the number of grams of cheese taken, and multiply the quotient by 100. This is the per cent of salt in the cheese.