ON THE ACTION OF AIR AND WATER UPON IRON. 19 
would be the most durable for ship-building ; the Low Moor comes next to 
these ; and to these again, the plates from South Wales. 
345. I now proceed to make some observations upon one or two of the 
methods of protection for iron which have been recently published, before 
giving the details of that which I have proposed. 
The whole of the various methods that have been from time to time pro- 
posed for protecting iron from corrosion, may be divided into two classes : 
those which protect the iron by a mechanical covering, more or less perfect, 
and itself not acted on by the corroding agent; and those wherein, by the 
contact of some other body, a change is produced in the electric or chemico- 
polar condition of the iron with respect to the corroding agents, such that 
they cease to be so with reference to 7. 
To the first class belongs the whole tribe of paints and varnishes, and 
every attempt to cover or sheathe the surface of the iron with another metal 
which is electro-negative to it. The principal methods of this class which 
have been recently patented, are those of Miles Berry (a communication), 
for coating iron with alloys of zinc and copper by cementation (May 1838, 
Newton’s Journal, conj. series, vol. xv. p. 91); Neilson’s (of Glasgow), for 
coating iron with brass, by dusting the interior of the mould with brass filings 
before the metal is poured in, &c., a process absolutely useless; and Joseph 
Shore’s, sealed March 1840 (Rep. Arts, No. 84, December 1840), for precipi- 
tating copper or nickel on iron by Spencer's electrotype process ; and Elking- 
ton and Ruolz’s process, already spoken of, patented in December 1840, which 
includes both classes of protection. 
To this class also Wall's (of Bermondsey) process; so much brought before 
the public, may be said to belong, inasmuch as (although, with reference to 
zine or copper, this process may produce a change in the chemical relations 
of those metals to air and water) it has no such effect upon iron, and merely 
acts upon it as an imperfect but most expensive paint or varnish. 
$46. The principle of the second class has been already fully pointed out 
in preceding Reports, and its conditions experimented on and stated. It has 
been stated that Sir H. Davy, Edmund Davy, Pepys and Sorel, long since 
invented or applied this sort of protection to iron. 
The principal inventions dependent upon this method which have been 
patented, are those by H. W. Crauford (sealed April 1837, Rep. Arts, N. 8. 
vol. ix. p. 289), and Fountainmoreau’s (sealed May 1838, Newton’s Journal, 
conj. series, vol. xvi. p. 289). These two patents are in fact one; they are 
essentially the same, differing only in certain details of application, &c. They 
both consist in the application of a thin coating of zinc to the whole surface 
of the iron, by dipping the iron into fluid zinc, when properly cleaned before- 
hand, or by coating it with a paint, or rubbing it, or lapping it up in a powder 
of metallic zinc. 
Of these various methods not one is completely effective, the causes of 
_ which have been already fully discussed throughout these Reports. 
347. The paint made of powdered zinc, and Wall’s mercurial paint have 
no efficacy of an electro-chemical kind whatever towards iron, They differ 
_ in no respect from any other paint in being towards # mere mechanical 
coverings, more or less perfect. The results of experiments on the zine paint 
have been already given. Wall's patent, which describes an absurd and 
roundabout process that reminds one of the recipes of the alchymists, and 
can scarcely be the result of chemical knowledge, is, when stripped of its 
useless encumbrances, simply a mode of making a mixture of several salts of 
iron and mercury, chiefly sesquichloride and sesquioxide of iron, and sub- 
~ chloride and subnitrate of mercury, with probably suboxide of mercury, which 
cQ2 
