Acidity in Milli, 
59 
neulralisecl the acidity of any given liquid ? It will be obvious 
that the whole exactness of our gauge will depend upon this 
point ; for if we add too much or too little alkali we shall intro- 
duce a source of error into our test which will vitiate it, and may 
make it practically useless. This difficulty is overcome by the 
adoption of a very simple device. Suppose that before adding 
an alkali to our acid liquid we place in the latter a piece of blue 
litmus-paper : the blue colour will, as before indicated, become 
changed into a red. Let us now cautiously add our alkali (to 
facilitate which we will dissolve it in water, so as to obtain a 
solution, which may be the more easily employed for neutralising 
purposes), and observe what takes place. At a certain stage 
of the operation the red colour of the litmus will begin to 
change into purple, and if more alkali is added the purple will 
pass again into the original blue. Here, then, in the stage 
which exists between the change of the red colour of the litmus 
into blue we have an indication of the stage of neutralisation ; 
and the more this stage can be shortened the more exact the 
determination of the point of neutralisation will be. 
From its property of thas indicating the point when neutrali- 
sation is reached in the mutual action of acids and alkalies on one 
another, litmus is technically called an indicator, and it belongs 
to a class of chemical agents all of which possess the projoerty of 
being affected in the same way, and consequently of being avail- 
able as indicators. Some of these bodies, unlike litmus, have no 
colour at all, except in the presence of either an acid or an alkali ; 
and this is an advantage in an indicator, as it is much easier for 
the eye to recognise the change from colour to no colour than from 
one tint to another, especially when that change is from such a 
colour as blue to another colour like red, through all sorts of 
intermediate gradations of purple. Moreover, litmus is not 
nearly so sensitive in its reactions to acids and alkalies as some 
other indicators are. Amongst these is a body known as 
;plienolphtliallein, one of the numerous members of that most 
interesting group of chemical products, the derivatives of coal- 
tar. This substance, which when in solution gives a full purple 
colour in the presence of a most minute quantity of any alkali, 
loses that colour altogether when the alkali is neutralised by an 
acid ; or, whilst in the presence of an acid it gives no colour at 
all, when the acid is neutralised by the addition of an alkali, 
the purple tint makes its appearance. By taking advantage, 
therefore, of these properties of this sensitive colour reagent, and 
adding a minute quantity of it to any acid solution, and then 
gradually adding to the mixture an alkaline solution until a 
