ON THE ACTION OF AIR AND WATER UPON IRON. oh 
$52. The method of preventing corrosion and “fouling” of iron vessels 
consists of three principal operations ; the first of which is designed to pre- 
vent corrosion, the second aids the former, and also prevents the first part of 
the process being afterwards interfered with by the third operation, which has 
reference solely to the prevention of “ fouling.” Hence, for all other articles 
of iron, except certain of those immersed in sea or fresh water, the first two 
operations are alone requisite, and in some cases the first only. I proceed 
to describe in detail the modes of performing the several operations. 
353. The first consists in covering the iron with a particular alloy of zine 
in fusion; for this purpose the iron surfaces require to be previously cleansed 
from adhering oxide. The boiler-plates, angle-iron, &c. to be used for ship- 
building should not be permitted to acquire any red rust previous to the 
operations about to be described; and if required to lay by for a considerable 
time previous to use, or to be transported to a distance, should be rubbed over 
with drying oil or other greasy matter to preserve them temporarily from rust : 
this oily coating may be afterwards removed by immersion in any alkaline ley. 
354. The plates or other pieces of cast or wrought iron or steel are to be 
immersed on edge (or in such a position that the detached scale of oxide can 
readily fall off), in a suitable vessel of wood, pottery, stone or lead, contain- 
ing dilute sulphuric acid (specific gravity about 1°30), or dilute hydrochloric 
acid, specific gravity about 1°06 at 60°, formed by diluting these acids re- 
spectively, as they are usually found in commerce, with rather more than an 
equal bulk of water. 
The diluted acid is best warmed, which may be conveniently done by a 
steam-jacket round the vessel, or by blowing steam into the acid, as it is de- 
sirable that the scale of oxide should be detached as rapidly as possible from 
the surface of the iron. The acid vessel, in operating on the great scale, is 
best formed so that the lower portion of acid and the scales which have de- 
posited can be occasionally withdrawn, to prevent waste of acid or increased 
length of time in the cleansing process. The iron must be wholly immersed, 
and the bubbles of gas formed on its surface must be free to ascend in the 
fluid and escape. 
$55. As soon as the scale of oxide has become detached or loosened from 
the iron, the plates or other pieces are to be removed from the “ cleansing 
bath” and washed with cold water. The surfaces are now to be thoroughly 
scoured by hand or by power, with sand or emery, or with pieces of grit 
stone, while exposed to a small running stream of water, until they appear 
quite clean, bright and metallic. 
_ 356. The plates or other articles of iron are now immediately, and without 
being permitted to dry, immersed in the “ preparing bath,” in which they are 
to lie until about to be covered with the alloy of zine to‘be hereafter de- 
seribed. The fluid which forms what may be called the “‘ preparing bath” is 
‘ made in the following way:—To a saturated cold solution of chloride of zine 
is to be added an equal bulk of a saturated cold solution of sal-ammoniac, and 
to the mixed solutions as much more sal-ammoniac in the solid state is to be 
_ added as they will dissolve; or these solutions may be made and mixed hot, 
_ and the solid sal-ammoniac then added, if thought more desirable, but the 
addition of more water on cooling is then requisite. The “ preparing bath” 
may also be formed of sulphate of zinc and sulphate of ammonia, or acetate 
of zine and acetate of ammonia, or of any other soluble double salt of zinc 
and ammonia, or salt of manganese and ammonia; the nitrates of zinc and 
ammonia are the least advantageous, but none answer the purpose so well as 
the above-described chloride of zine and sal-ammoniac. No free acid should 
be present in these solutions. 
