60 
Acidity in Milk. 
permanent purple colour is produced, we are enabled to recog- 
nise with ease the precise point when the acidity is perfectly 
neuti'alised. 
Having thus arrived at a knowledge of how to measure 
acidity, it only remains to decide what scale of measurement we 
shall adopt for the purpose — that is to say, what unit of measure- 
ment we shall assume. Just as in measuring the size of a can 
of milk we take an inch for our unit, and as in estimating the 
temperature of the milk we take a degree on the thermometric 
scale, whatever that scale may be, as our unit of temperature, so 
in gauging the acidity of the milk we require to decide on some 
deBnite quantity of acidity which can be itself measured with 
certainty whenever we want to reproduce it, and in terms of 
which we can express or estimate the acidity of any given liquid. 
Now the inch, and also the degree of the ordinary thermometric 
scale used in this country (Fahrenheit's), are both of them 
arbitrary units ; that is, they were originally adopted for reasons 
of practical convenience rather than to satisfy theoretical con- 
siderations. On the same grounds, for reasons which it is un- 
necessary to discuss here, the unit of acidity which the writer 
has adopted is one which he has found most convenient for the 
general objects which have to be kept in view in measuring 
acidity for dairy purposes. It is represented by an exceedingly 
minute weight of pure, dry, crystalline oxalic acid, an acid 
which is generally selected by the chemist for this purpose, 
because it can be easily obtained of uniform strength. It is 
simply necessary to make a solution containing a definite quantity 
of this acid, and then to make an alkaline solution which exactly 
equals it in strength, and we then know that every unit of the 
alkali represents one unit of our normal or standard acid ; and 
if we can only measure that unit with ease and accuracy we have 
at our disposal a mode of measuring acidity which is, in theory, 
as exact and as reliable as the foot-rule or the thermometer, 
and is in every way comparable with them. 
Now here comes in one of the practical difficulties of this 
question. It is very easy for the chemist to measure acidity in 
his laboratory, because he has not only the appliances and 
materials at his disposal for so doing, but the knowledge and 
skill which are necessary for using them, and for avoiding the 
fallacies into which an untrained person may easily fall in so- 
doing. In order, in the first place, to meet the requirements of 
the problem, it is necessary to have as a neutraliser an alkaline 
solution of considerable strength, or otherwise the delicacy of the 
test would be proportionatc^ly impaired and its practical value 
in the same degree diminished. But, unfortunately, the alka- 
