PROTECTION OF IRON AND OTHER METAL WORK. 195 
be regarded as a medium at all, but rather as a diluent 
for linseed oil. 
Another point deserving of attention is the question of 
the expansion of paint, which is very little thought of, and 
it should be borne in mind the difference in action in this 
respect between a varnish and a paint, the former shrinks 
while the latter expands. For a metal like iron that is 
used in positions where alternate expansion and contraction 
is frequent and inevitable, the most adherent and the most 
unchanging in regard to volume changes will obviously be 
the best. On the domestic hearth iron work has been pro- 
tected for centuries by the application of the solid carbon 
popularly known as ‘blacklead,’ an instance of solid paint 
all pigment of body without either vehicle or medium, being 
simply rubbed on with a brush. But black lead, red lead, 
and white lead are very well in their place and under cer- 
tain conditions there seems to be no universal covering 
composition that wiil be universally suitable for the pro- 
tection of iron. Red lead I think, when it can be obtained 
free from adulteration, leads the record in practical value; 
and the engineer to the Water and Sewerage Board tells 
me that great success has followed the use of red lead and 
linseed oil. 
One of the most striking instances of the corrosion of 
iron work that has come under my notice is that of the 
wearing away of the casing of one of our artesian bores. 
The conditions so far as I know them are as follows :—the 
water is alkaline owing to the presence of carbonates and 
bicarbonates, the temperature of the water at the moment 
of flow is 95° F., and along with the water, sand and large 
pebbles are brought up to the surface by the impetuous 
uprising water. Common salt and bicarbonate of iron, with 
abundance of free carbonic acid is present. Among the 
gaseous constituents are hydrocarbons, methane, free 
