
ON THE ACTION OF AIR AND WATER UPON IRON. 23 
the pieces of iron, as these defend it from the action of the atmosphere. It 
is desirable that the melting vessels should be as deep, and expose as small a 
surface, as the nature of the articles to be immersed will allow. At the mo- 
ment of immersion of a plate or other article of iron, the surface of the alloy 
is to be cleared of all dross or oxide by a wooden skimmer. 
361. As soon as the iron is withdrawn from the alloy it is to be plunged 
into cold water and well washed therein. The surface of the iron is now in 
a condition to resist corrosion. 
362. By the addition of a larger portion of mercury to the before-mentioned 
alloy of zine, mercury and sodium, or potassium, cast or wrought iron or steel 
may be coated therewith at a lower temperature, or even cold, by simple con- 
tact accompanied with friction; but a smaller quantity of mercury than that 
before given will often be found most convenient. 
363. In the case of iron ships the foregoing operations are best performed 
upon the plates and ribs, after they have been all bent and fitted to their 
places, and the plates riveted together into large pieces of eight to ten feet 
square or more, which, when again put “ into frame,”/or placed in their re- 
spective positions in the ship’s hull, are to be united by rivets countersunk 
from the outside, and hence closed inside the vessel. The countersunk heads 
of these rivets should be also coated with the alloy; and I have mentioned in 
detail elsewhere how these may be heated for riveting without injuring the 
alloyed head. 
364. The hull of the iron vessel being thus completed and wholly covered 
with the alloy, is now to receive a coat of varnish all over of the composition 
about to be described, and which is best laid on with a spatula, or thin 
flexible blade of iron, as a brush produces minute air-bubbles, which leave 
spaces uncovered on the drying of the varnish. 
The varnishes described will dry or get hard and coherent at ordinary 
temperatures, but where convenient it is desirable to expose them for some 
hours to a temperature of about 300° Fahr., which gives them greater ad- 
hesion and durability. 
~ 365. To form the varnish No. 1, take 50lbs. of foreign asphaltum, melt 
and boil it in an iron vessel for three or four hours, adding gradually, in fine 
powder, 16lbs. of red lead and litharge ground together in equal proportions, 
with ten imperial gallons of drying linseed oil; bring all to a boiling tempe- 
rature, melt in a separate vessel 8lbs. of gum anime (which need not be of 
the clearest or best quality), add to it two imperial gallons of drying linseed 
oil boiling, and 12lbs. of caoutchouc softened or partially dissolved by coal- 
tar naphtha (as practised by the makers of waterproof clothes); mix all to- 
-gether in the former vessel and boil gently until, on taking some of the 
varnish between two spatulas, it is found tough and ropy. When this 
“body” is quite cold, it may be thinned down with from thirty to thirty-five 
gallons imperial of turpentine or of coal naphtha. 
This is the best varnish I am acquainted with for the purpose of covering 
iron ; it is not acted on when dry and hard by any moderately diluted acid 
or caustic alkali; it does not, by long immersion, combine with water, and 
form a white and partially soluble hydrate, as all merely resinous varnishes 
and all oil paint do, and it is so elastic that a plate covered with it may be 
‘bent several times without its peeling off; and lastly, it adheres so-fast that 
nothing but a sharp-edged instrument will scratch it off the surface of iron. 
_ The varnish No. 2 is of a cheaper sort, but not quite so good. Common 
coal or gas tar is to be boiled in an iron caldron at so high a temperature 
that the smoke from it is of a yellow dun colour, or the tar is to be caused to 
flow through red-hot iron tubes. The boiling is to be continued until the 
