138 On the Composition and Proj)erties of Drinking- Water, 
Composition of an Ina-ustation taken from an Iron Water-pipe. 
Grains. 
Water of combination and a little organic matter) 7-93 
(loss on heating) j * 
Oxide of iron 88-89 
Carbonate of lime '18 
Silica -Gl 
Carbonic acid, magnesia, traces of sulphuric acid,! 9.39 
manganese, zinc (not determined separately) .. j " 
100-00 
On further enquiry I learned that the galvanized-iron tank 
Tvhich supplied the water was corroded to a certain extent, and 
a deposit similar to the material in the pipes was found at the 
bottom. The composition of the brown incrustation clearly 
betrayed its origin. It was mainly hydrated oxide of iron, 
formed by the action of the water upon the galvanized-iron tank. 
The tank deposit evidently was carried down in the supply- 
pipe, and gradually choked it up. The pipe itself, although 
rusty in the interior, did not appear to have been eaten away in 
any appreciable degree ; the substance which choked it could 
not therefore have been derived entirely from the action of the 
water upon the pipe, and as the lake-water on analysis was found 
to contain merely traces of oxide of iron, it could not have given 
rise to the deposit, which was evidently carried down mechani- 
cally from the tank where it was first produced. 
Probably the iron supply-pipes would have remained in good 
working-order for many years but for the iron tank. In order 
to remedy the inconvenience arising from the storage of soft 
lake-water in galvanized-iron tanks, I recommended my corre- 
spondent to replace the tank by a slate cistern, and have no doubt 
by this means the supply-pipes will be kept clear from any 
deposit and in good working-order for years to come. 
Hard Water. 
Springs rising in granitic regions, or in localities where primi- 
tive rocks, little acted upon by water, prevail, or which have 
their origin in siliceous strat;i, furnish soft water, Ave have seen. 
On the other hand, springs which rise in the oolite or chalk-for- 
mation, and all waters which flow over calcareous rocks, or pass 
through soils abounding in lime, are always more or less largely 
impregnated with carbonate and sul])hate of lime and magnesia. 
It is chiefly to the lime and magnesia, in combination with 
carbonic or sulphuric acid, that what are called hard waters owe 
their property of curdling soap. Perfectly pure or soft water, in 
