5S 
Acidity in Milk. 
a piece of blue litmus-paper is dipped into a sample even of fresh 
milk it gradually turns purple, and if the milk is stale the purple 
will change into a bright pink. By the rapidity with which 
this change takes place we may form a rough idea as to whether 
the milk is very acid or not, as we may do also by comparing 
the indication given by the milk with that given by vinegar. 
But even litmus-paper is insufficient, for want of delicacy, to 
indicate slight variations in acidity, or to give us any precise 
idea of the relative acidity of any two acid liquids which we 
may wish to compare with one another. For such a purpose we 
require a gauge, which we can apply to the acid liquid in the 
same way as we use a foot-rule to measure cubical capacity, or 
a balance to measure weight, or a thermometer to measure 
temperature. How is this to be obtained ? 
To construct such a gauge we have to take advantage of a 
chemical fact which is familiar to everyone, and that is, that 
the property of acidity which ordinary acid bodies jiossess can 
be neutralised and concealed by the addition to them of bodies 
of an antagonistic character, known as alkalies, such as common 
washing-soda. A fact which is not quite so familar to those 
who have no knowledge of cliemistry, but which a little simple 
experimentation will make very clear, is, that if we take a definite 
quantity of any acid liquid, say one ounce of pure vinegar, we 
shall find that, however often we repeat the experiment, it will 
always take exactly the same weight of any given sample of 
washing-soda to neutralise the acidity. Here, then, we have a 
basis for the construction of a gauge of acidity. For all that we 
have to do is to settle on a standard quantity of soda, and use it 
as a means of expressing the acidity of any liquid to which we 
may wish to apply it. 
What we need for this purpose is a iinit weight, just as we 
take the inch as the unit of length and the degree as the unit of 
temperature. We ma}-, then, call the amount of acidity which our 
unit weight of alkali will exactly neutralise one degree ; and it 
will be obvious that by thus taking as niany unit Aveights of 
alkali as may be necessary to neutralise the acidity of a definite 
quantity of any acid liquid we can speak of its acidity as being 
5, 10, or any other number of degrees, as the case may be. If 
we always use the same unit weight of alkali and the same bulk 
of liquid we shall be able to compare the acidities either of the 
same liquid at different times, or of different liquids, with the 
same ease and certainty that we can measure the contents of 
two different cans of milk, or estimate their temperatures at the 
same or at different times. 
But, it may be asked, How are we to know when we have 
