20 BULLETIN 718, U. S. DEPARTMENT OE AGKECTTLTUKE. 
The extent of the heating surface of a boiler depends on the length 
and diameter of the shell and the number and size of the flues. It 
is customary in calculating the heating surface of the shell to con- 
sider that two-thirds of it and the entire surface of the flues is ex- 
posed to the action of the heat. 
STEAM BOILEK WATEH. 
A steam boiler needs good water as much as it needs good fuel. All 
water used in boilers contains more or less impurities and acid. Im- 
purities which cause trouble include soluble salts of calcium and mag- 
nesium, bicarbonates of alkaline earths, and sulphate of lime. Water 
containing more than 0.005 per cent of free sulphuric or nitric acid 
is likely to cause serious corrosion, and more than one-tenth of 1 per 
cent of acid will cause scale. Hard water invariably forms scale, and 
comparatively soft water may also do so if the boiler is used too long 
without being emptied. Foaming is caused chiefly by an excess of 
alkaline salts, which causes the water to form suds as if soap had 
been placed in it. 
Scale not only decreases the efficiency of the boiler but also causes 
deterioration : for. when it is sufficiently thick the conducting power 
of the boiler is reduced, and the tubes and plates become overheated 
and crack or burst. Again, the scale may keep the water from sec- 
tions of the heated plates for some time, and then give way. causing 
large volumes of steam to be suddenly generated, possibly resulting 
in an explosion. It has been demonstrated that J of an inch of scale 
in boilers causes a loss in heat transmission of from 10 to 12 per 
cent, and this loss of heat increases with the thickness of the scale. 
A porous scale retards the heat transmission more than a solid scale. 
It sometimes happens that different kinds of scale may be found in 
the same boiler, owing to the different temperatures of the sheet in 
different parts and to the circulation of the water. The scale on the 
tubes is also different from that on the sheets, owing to the same 
causes. 
Methods for the purification of feed water consist, of the use of 
feed-water heaters, scum catchers, and blow-off valves, or chemicals 
placed in the boiler or in the water before it reaches the boiler. Puri- 
fying chemicals placed in the boiler are soda ash, caustic soda, phos- 
phate of soda, tannin compounds, fluoride of soda, and aluminate of 
soda. As a rule, the expense of purifying feed water with chemicals 
makes it prohibitive for small mills. The use of feed-water heaters, 
scum catchers, and blow-off valves, however, is strongly recommended. 
To prevent the adherence of scale to the boiler shell, many sub- 
stance^ have been used, such as potatoes, kerosene, and other remedies, 
organic and mineral. Of the boiler compounds found on the market 
none have given more general satisfaction than those which have soda 
