TABLE SALT 271 
particles have been broken up so fine that we cannot see 
them. Evaporate the solution over a spirit-lamp or bunsen 
burner and show resulting deposit. Question pupils on 
this operation until they are thoroughly familiar with what 
has happened. 
Heat a small quantity of Salt in a crucible over a hot 
flame, and find out that it is fusible (see lesson on Rock- 
Salt), but does not burn. 
Leave some Salt exposed to the air in an open vessel— 
and observe that it has become quite moist. Lead pupils 
to see that the moisture must have come from the air— 
and hence to recognise the necessity of keeping Salt in a dry 
place and of covering up the vessels containing it. When 
making this test, the Salt to be operated on should be 
placed in a damp situation the evening before taking the 
lesson. In dry weather of course the amount of moisture 
in the air is so small as not appreciably to affect the ex- 
posed Salt. 
Call attention to the vessels in which Salt is usually kept. 
Why are these made of wood and furnished with lids? 
Salt rapidly absorbs moisture, which is the great vehicle for 
the carriage of oxygen. The moist Salt coming in contact 
with a metal such as tin causes rapid oxidation; hence a 
tin vessel containing Salt soon becomes rusty and dis- 
coloured. Wood not only escapes oxidation, but is able 
to absorb some of the moisture from the Salt ; the contents 
of the box are in this way kept clean and dry, 
Where and how obtained. Salt is obtained from 
mines, from Salt or brine springs, and from sea-water. 
When obtained from mines, the Rock Salt is brought to 
the surface, crushed and dissolved. The impurities are 
then removed by processes which are too complicated to 
be discussed here, and the pure Salt is recovered by 
evaporation. 
