24 REPORT—1843. ; 
residue is a solid asphaltum, breaking with a pitchy fracture. It is essential 
that the boiling should be carried on at this high temperature, as the perma- 
nency of the varnish in water depends upon the tar having been submitted 
to the temperature at which naphthaline is formed by the decomposition or 
breaking up of the original constitution of the tar. 
Take 56lbs. of this coal-tar asphaltum, melt it in an iron vessel, add ten 
imperial gallons of drying linseed oil ground with 25lbs. of red lead and 
litharge in equal proportions; add to the whole, when well mixed, and after 
boiling together for two or three hours, 15lbs. of caoutchoue, softened or 
partially dissolved by coal naphtha (as before described) ; when cold, mix 
with twenty to thirty gallons of turpentine or coal naphtha, and the varnish 
is ready for use. 
366. Either of these varnishes is to be applied over the whole surface of 
the iron and suffered to dry ; and as this forms the final preservative coat upon 
all articles except iron ships, buoys, &c. requiring to be preserved from 
“ fouling,” any desirable colour may be given to it by colouring materials, 
which should be peroxides not acted on by air and water. 
367. The last operation, viz. that to prevent “ fouling,” now remains to be 
performed upon ships, &c.; for this purpose a strong-bodied thick paint is to 
be made with drying linseed oil, red lead and sulphate of barytes (or white 
lead may be used, but not so advantageously), and a little turpentine. To 
every 100lbs. of this paint, when mixed, is to be added 20|bs. or thereabouts 
of oxychloride of copper and 3lbs. of a mixture composed of hard yellow 
soap, melted with an equal weight of common resin and a little water. 
The colour sold in commerce originally under the name of Brunswick green 
was an oxychloride of copper. The Brunswick green of commerce at present 
is a different thing, but the oxychloride of copper may be obtained at a cheap 
rate by various known methods, which it is unnecessary to detail. With this 
paint the whole immersed hull of the vessel is to be coated over the before- 
mentioned varnish ; it must then be permitted to dry and stiffen for three or four 
days before the ship is floated out of dock. The operations are now completed, 
and the hull of an iron ship so treated will resist “ corrosion” and “ fouling.” 
368. The principles upon which this method of protection rests may be thus 
stated :—By the use of the “ preparing bath,” the surface of iron, of whatever 
sort, is more effectually cleansed than has before been practicable, and all 
minute particles of foreign matter removed from the surface ; and by the re- 
actions which take place in the “ preparing bath,” in which metallic amides 
are formed, and hydrogen evolved at the surface of the iron, a powerful ten- 
dency is given to the iron to combine with other metals. Again, by the pre- 
sence of the small quantity of sodium or potassium in the alloy, a greatly in- 
creased tendency to combine with iron is conferred upon 7¢, while any minute 
portions of oxide, either suspended in the fluid alloy, or which have escaped 
the previous operations upon the surface of the iron, are reduced to metal. 
Thus both metals are presented to each other in a state of absolute purity and 
in the most favourable circumstances for combination. 
369. But further, when the alloy, or generally any alloy of mereury with 
metals electro-positive to it, is exposed to the action of a solvent, the positive 
metal at the surface is first acted on, and the surface becomes shortly covered 
with pure mercury. The result, therefore, of the primary reaction of air and 
water on this alloy is, that the coating of the iron becomes covered with a 
very thin film of amalgemated zinc, which is known not to be acted on by 
fluid menstrua, except under peculiar conditions. I found no caleareous 
coating formed on such a surface in sea or fresh water. The varnish laid over 
this is intended as a sheathing, to give additional durability and mechanical 
