Let us make our statement clear by a simple experiment. To a little fresh milk we add a weak acid, the milk curdles at once and by dipping a piece of litmus paper (obtained at any chemist’s) into the mixture, it will turn red, showing the presence of acid. Litmus, by the way, is obtained from a lichen; in the presence of acid it is red, an alkali, the opposite of an acid, turns it blue. In a neutral solution, that is to say one that is neither acid nor alkaline, litmus is of a purplish hue. To continue our experiment, we allow another sample of the same milk to stand for a day or two in a warm, dark place and again the milk will be curdled. A test with the litmus will show that the solution is acid. The bacteria themselves have not curdled the milk but they have liberated a substance, called a ferment, which has split up part of the milk into an acid, amongst other things and that acid has actually done the curdling. For this reason, weak alkalies are sometimes added to milk. Acids and alkalies, of equal strength form neutral solutions, so that, when the milk bacteria begin their activities which result in the formation of acid, it is at once made neutral by the alkali. By this means, curdling is postponed for a little while, though there comes a time, of course, when all the alkali is used up, then the acid gains the upper hand and curdling takes place. We could if we wished continue adding more and more alkali to keep pace with the formation of acid, but too much alkali would be as unpalatable as too much acid, so nothing would be gained.