68 TEST PAPERS. 



and this property is therefore made use of by the 

 chemist, to detect the presence of a free acid ; for this 

 power of acid ceases immediately, when they are 

 combined with bases : because their powers are then 

 neutralized (110). 



114. A very common illustration of this change of 

 color may be seen in the pickling of red cabbage. 

 Every one knows that red cabbages, as they are called, 

 are really dark purple or blue, whilst growing, and 

 they only become red by the action of the acid vin- 

 egar employed in pickling them; the same effect 

 would be produced if any other acid was employed. 



115. If now we add to some cabbage thus reddened 

 by acid, a little alkali, either potash, soda, or ammo- 

 nia, or indeed a portion of any base, the color will 

 soon be restored to its original blue ; because the 

 acid is neutralized by the base. And if the base 

 employed be one of the three alkalies, or their car- 

 bonates, such as common pearlash, which is the car- 

 bonate of potash, the solution of cabbage will become 

 green, because free alkalies, and likewise their car- 

 bonates, have the power of turning vegetable blues 

 green. 



116. Alkaline solutions have also the property of 

 turning certain vegetable yellows red, such, for ex- 

 ample, as common turmeric ; these tests, as they are 

 termed, are very easily applied, and papers stained 

 with blue or yellow vegetable colors, are consequently 

 most useful indicators of the presence either of free 

 acid, or free alkali, in a solution. 



