136 APPLIED SCIENCE 



has no effect. Such a condition is wholly due to the action 

 of the dissolved solids upon the soap. Hard water contains 

 either magnesium or calcium sulphates or carbonates. If 

 carbonates only are present in the water, it is likely to become 

 soft when it is boiled, for boiling drives out carbonic acid gas 

 (CO 2 ) which holds the carbonates in solution. 



Thus we see that water may differ in its properties accord- 

 ing to the influence of substances to which it has been ex- 

 posed. Rain and snow water are difficult to obtain; river 

 water may be muddy, more especially in stormy weather; 

 artesian well water will contain in solution the minerals with 

 which it has come in contact. The source of the purest 

 water is a location near a mountain, or in a mountainous 

 country. There the upland surface water has not yet come 

 in contact with impurities, and has had little opportunity 

 to dissolve lime or magnesia compounds. 



145. Importance of Acids and Alkalies. In addition to 

 water, the most important compounds or substances used 

 in chemical changes are acids and alkalies. They may be 

 called the fundamental chemical agents that produce chemi- 

 cal changes. It is important to know their properties. 



146. Nature of Acids. An acid is a compound of hydro- 

 gen with a non-metallic element or a group of elements that 

 act as one, called a radical. The acid may be a gas soluble 

 in water, as muriatic acid, or a liquid, such as sulphuric acid, 

 or a solid, such as oxalic acid. All acids have a sharp, sour 

 taste and most of them act on metals. The test used for 

 determining whether or not a solution is an acid is to place 

 a drop of the solution on a piece of blue litmus paper (paper 

 dyed blue with the juice of a small plant). If the blue color 



