132 EXPERIMENTAL GENERAL SCIENCE 



113. Hard Water. Hard water is simply water carrying 

 certain mineral compounds in solution. The substances 

 usually found are calcium sulphate, or gypsum (CaS0 4 ), 

 magnesium sulphate (MgS0 4 ) and calcium hydrogen car- 

 bonate (Ca (HC 0)3)2), which, instead of producing an agreeable 

 lather with soap, form a scum that fails to cleanse. It is, 

 therefore, customary to add borax, ammonium carbonate, or 

 washing soda which combines with these substances and thus 

 " softens" the water. Water containing calcium hydrogen 

 carbonate is said to be temporarily hard because it can be soft- 

 ened by boiling, or by adding slaked lime. This causes the 

 dissolved material to settle to the bottom. In steam boilers 

 when hard waters are used, this accumulation forms a 

 " scale" which causes much trouble by preventing the proper 

 heating of the water. Permanently hard water contains 

 calcium sulphate and cannot be softened by boiling. 



114. Diffusion. If a lump of a soluble substance be dropped 

 into a liquid, it will ultimately spread evenly through it, even 

 if the liquid is not stirred. The process by which this is ac- 

 complished is known as diffusion. A liquid may dissolve in a 

 liquid, or a gas dissolve in a gas, by diffusion, as when illumi- 

 nating gas escapes from leaking pipes and spreads through a 

 room, or as water vapor disappears in air. In such cases, 

 either gas or liquid may be considered as the solvent, though 

 it is customary to regard the one present in the larger amount 

 as entitled to the name. When a weak and a strong solution 

 of the same kind come into contact, they also mix by diffusion. 



115. Other Forms of Solutions. While the disappearance 

 of a solid in a liquid is the most familiar form of solution, there 

 are various other associations of substances that are fairly 

 included under the title. Gases as well as solids may be dis- 

 solved in liquids. The air breathed by fishes and other aquatic 

 animals is thus dissolved in the water. Gases may even be 

 dissolved in solids. Charcoal will take up in this way 40 



