POTASSIUM AND SODIUM. 215 



world. It all originally comes from the mineral world, and 

 being absorbed from the soil by plants, through them it 

 gets into the blood of animals by their food. What its 

 special uses are in animals, beyond the fact that no food can 

 be digested without it, we know not; but that it is essen- 

 tial its constant presence in the blood shows. Salt is suffi- 

 ciently soluble for all practical purposes. It does not deli- 

 quesce easily, troubling us in this respect only when the air 

 happens to be very damp. Unlike most other salts, it dis- 

 solves almost equally well in cold and hot water. It is 

 scarcely soluble at all in alcohol. It crystallizes in the,form 

 of cubes. Sometimes the crystals have 

 an arrangement which is hopper-shaped, 

 as represented in Fig. 89. This is be- 

 cause that which is first formed on the 

 surface sinks a little in the solution, and 

 then there is an addition upon its outer Fi s- 



edge all around; and this goes on continually, the outer edge 

 all the time enlarging, and the solid salt all the time sink- 

 ing as it increases. The upper edge is during the whole 

 process just at the surface, evaporation adding continually 

 to it. 



297. Decomposition of Salt. Salt is decomposed in many 

 chemical operations, but its elements are so firmly united 

 that it is by no means easily decomposed. Heat, for ex- 

 ample, can not drive off the chlorine, as it does carbonic 

 anhydride, from limestone. Most of the substances with 

 which salt is apt to come in contact can not decompose it. 

 Strong as is the attraction of oxygen for sodium, it can not 

 take it away from chlorine. If now, on the other hand, salt 

 were easily decomposed, as it circulates by means of water 

 constantly among a great variety of substances in the sea, 

 in the soil, and in the fluids of vegetables and animals it 

 would be a source of continual danger. The evolution of 



