220 MAWS CONQUEST OF SUBSTANCES 



In order to understand the action of several of these sub- 

 stances we must study the properties of two groups of chemi- 

 cals known respectively as acids and bases ; the first of 

 these may be represented by* vinegar, sulphuric acid, and 

 oxalic acid ; and the second, by ammonia, lye, and limewater. 



202. Acids. All of us know that vinegar and lemon juice 

 have a sour taste, and it is easy to show that most acids are 

 characterized by a sour taste. If a clean glass rod is dipped 

 into very dilute acid, such as acetic, sulphuric, or nitric acid, 

 and then lightly touched to the tongue, it will taste sour. But 

 the best test of an acid is by sight rather than by taste, be- 

 cause it has been found that an acid is able to discolor a 

 plant substance called litmus. If paper is soaked in a litmus 

 solution until it acquires the characteristic blue hue of the 

 plant substance, and is then dried thoroughly, it can be used 

 to detect acids, because if it comes in contact with even the 

 minutest trace of acid, it loses its blue color and assumes 

 a red tint. Hence, in order to detect the presence of acid in 

 a substance, one has merely to put some of the substance 

 on blue litmus paper, and note whether or not the latter 

 changes color. This test shows that many of our common 

 foods contain some acid ; for example, fruit, buttermilk, sour 

 bread, and vinegar. 



The damage which can be done by strong acids is well 

 known ; if a jar of sulphuric acid is overturned, and some of 

 it falls on the skin, it eats its way into the flesh and leaves 

 an ugly sore ; if it falls on carpet or coat, it eats its way 

 into the material and leaves an unsightly hole. The evil re- 

 sults of an accident with acid can be lessened if we know 

 just what to do and do it quickly, but for this we must have 

 a knowledge of bases, the second group of chemicals. 



203. Bases. Substances belonging to this group usually 

 have a bitter taste and a slimy, soapy feeling. . For our present 



