78 SCIENCE PRIMERS. [MATERIAL 



temperature, so that the conversion of salt from the 

 solid state into the liquid state by solution in cold 

 water is obviously a very different process from lique- 

 faction by heat. Nevertheless the result is the same 

 so far as the condition of the salt is concerned. The 

 cohesion between its molecules is destroyed, and they 

 distribute themselves evenly among the molecules of 

 the water, just as the molecules of steam distribute 

 themselves among the molecules of air. And, when 

 you study chemistry, you will learn how it may be 

 proved that the smallest drop of the solution of salt 

 contains exactly the same proportion of salt as the 

 whole does. 



If brine is allowed to evaporate slowly, the mole- 

 cules of the salt arrange themselves, as the water 

 leaves them, in beautifully regular cubical crystals. 

 You may see them form easily enough if you watch a 

 drop of brine gradually dry up under a microscope. 

 The salt crystals contain nothing but salt. If they 

 are heated till they become red-hot they pass into the 

 fluid state; and when still further heated, the fluid 

 salt becomes a vapour or gas and, as such, flies off 

 into the air, or volatilizes. 



Thus we see that when salt and water are brought 

 into contact, the salt undergoes a certain amount of 

 change, while the water does not remain wholly un- 

 changed. For brine no longer boils at 2 12 but re- 

 quires a considerably higher temperature. The salt, as it 

 were, holds the water back and prevents it from assum- 

 ing the gaseous state under the same conditions as if it 

 were pure, just as, in the previous case, the water held 

 the alcohol back ; or we may say that the force of heat 

 which drives the molecules of liquid water apart, when 



