24*0 RAIN WATER. ICE AND SNOW WATER. 
Water cannot be obtained in any state of perfect 
purity except by the artificial process of distillation. 
ORDER L COMMON WATER. 
274. RAIN WATER is considered to be next in purity to 
distilled water, from its having undergone a natural distil- 
lation. Its foreign contents vary according to the state of the 
air through which it falls. Hence, for instance, when it 
passes through the atmosphere of a smoky town, it becomes 
impure: and when collected in towns, it frequently acquires a 
.small quantity of sulphat of //w?e(l92), and calcareous matter 
from the mortar and plaster of the houses. 
This water is always very soft ; and is, consequently, 
well calculated for the dissolving of soap, in washing 
and other processes. It is also peculiarly adapted to 
the solution of alimentary or colouring matter, in the 
preparation of food and dyeing, and is accordingly used 
to great extent for these purposes. By the addition of 
a small quantity of a solution of barytes (195), it may 
be rendered sufficiently pure for all chemical uses. 
If rain water be long kept, especially in hot climates, 
it acquires a disagreeable smell, and becomes putrid, 
and full of animalculse. 
275. ICE and SNOW WATER are equal to rain zcater 
in purity ; and the air having been expelled from them during 
the process of freezing, they are consequently devoid of air 
when first melted. 
Ice and snow, in their natural state, are highly im- 
portant to mankind. It is a general law of nature that 
all bodies become more dense and heavy by exposure 
to cold ; but the freezing of water is an exception to 
this law, and for a purpose of extreme benefit to man- 
kind. By this ordination it is that ice always rises to 
