268 NEW ZEALAND NATURE-STUDY BOOK 
soluble. Drop some more of the powdered mineral into 
the same vessel, and go on adding fresh supplies until the 
water ceases to dissolve it. Add a little more of the 
powdered substance so that the class can plainly see the 
deposit on the bottom of the vessel. Now place the 
beaker over the flame of a spirit-lamp or bunsen burner 
and heat gently—observe the gradual solution of the 
mineral. Salt is more soluble in hot water than in 
cold. Allow the solution to cool and notice the deposit 
of salt as the water cools. Hot water will hold more 
salt in solution than cold water. 
After dissolving a sufficient quantity in cold water— 
filter the solution and obtain a clear liquid. Place this in 
a beaker over a bunsen burner, and by gradually driving 
off the water recover the dissolved mineral. Question 
pupils until they are quite clear as to what has taken 
place in each of these operations. 
Origin. Rock Salt has been formed by the evaporation 
of very salt water in enclosed basins, a process now going 
on in many salt lakes, ¢.g., Dead Sea, Great Salt Lake of 
Utah, Caspian Sea, Sea of Aral. From these lakes or 
inland seas there are no outlets, and the rivers flowing into 
them are constantly bringing down Salt in solution—hence 
the lake waters are becoming increasingly saline. In some 
of them the water-area is diminishing with the formation 
of lagoons round their shores. From these the water is 
being removed by evaporation, with the result that a 
constant deposit of Salt is taking place. 
The Rock Salt of commerce has in ages past been 
formed in this way. The enclosed basins of Salt were 
gradually dried up, leaving deposits of the mineral 
behind. The areas containing these then subsided, and 
were covered up with material brought down by rivers, or 
spread out by the sea. 
