+81. The acid test+[26] is made by titrating a known amount of milk (Fig. 9) against an alkali solution of known strength, using phenolphthalein as an indicator. The object of the indicator is to tell the condition of the milk, whether it is acid, alkaline or neutral. The indicator does not change in an acid solution but turns pink when the solution is or becomes alkaline. To make the test, a known quantity of the material to be tested is placed in a white cup, and to this several drops of indicator are added. As an indicator, a 1 per cent solution of phenolphthalein in 95 per cent alcohol is commonly used. As an alkali solution, sodium hydroxide (NaOH) is used in the standardized strength usually either tenth (N/10) normal or twentieth (N/20) normal. This solution should be obtained in some one of the standardized forms commercially prepared. The alkali is added, drop by drop, from a graduated burette until a faint pink color appears. This shows that the acid in the milk has been neutralized by the alkali. The amount of alkali that has been used can be determined from the burette. Knowing the amount of milk and alkali solution used, it is easy to calculate the amount of acid in the substance tested. The results are usually expressed either as percentages of lactic acid or preferably as cubic centimeters of normal alkali required to neutralize 100 or 1000 c.c. of milk. This kind of test is on the market under different names, such as Mann's, Publow's, Farrington's and Marschall's. [Illustration: _FIG. 9._--An acid tester.] +82. Rennet tests.+--Several rennet tests have been devised, but the one most widely used is the Marschall (Fig. 10). This consists of a 1 c.c. pipette to measure the rennet extract, a small bottle in which to dilute the extract, a special cup to hold the milk and a spatula to mix the milk with the rennet extract. This cup has on the inside from top to bottom a scale graduated from 0 at the top to 10 at the bottom. There is a hole in the bottom to allow the milk to run out. [Illustration: FIG. 10.--Marschall rennet test.]