246 REPORT—1840. 
They are of very limited application, and appear to present a 
good deal of technical difficulty. 
204, These somewhat scattered facts, in the chemical history 
of paints and preservative varnishes, are little more than suffi- 
cient to show us the barrenness of this region of art, which has 
received no cultivation as yet but that of continued tentation on 
the part of the workman, undirected by scientific principles. 
Much might be hoped for, important in technical results, by the 
enlargement and correction of our still defective knowledge of 
the organic chemistry of the fixed and volatile oils, the resins 
and the bitumens. ‘To paints or varnishes alone, however, we 
are not to look for the means of complete protection from cor- 
rosion for oxidable metals; their liability to removal by slight 
external forces precludes this; their proper place, as mechani- 
cal protectors, will be found subsidiary to those which are de- 
pendent on chemical or electrical relations ; and one of their 
most important uses will probably be found in their application, 
in union with substances poisonous to animal and vegetable life, 
to the bottoms of iron ships, to prevent the “ fouling’’ produced 
by their accumulation, and now found of so much incon- 
venience. 
205, At the period of publication of the previous report, the 
preservation of cast and wrought iron, by the electro-chemical 
action of zinc, was beginning to excite that attention which 
was first drawn to it by the views of Sir Humphry Davy, and 
the subsequent experiments of Prof. EK, Davy; but there had 
not been time to enable any very decided results to be given in 
that report. I am now, however, in a condition to state the 
results of a tolerably complete train of experiments made on 
the protective powers of zinc, to iron and steel under various 
circumstances, some of which have been continued for upwards 
of two years. 
The experiments I have made on the electro-chemical power 
of protection of zinc to iron are divided into two great classes— 
those in fresh water and those in sea water; and each of these 
classes again divides itself into two, namely, those made with 
the preserved and preserving metals submerged to a greater or 
less depth in the fiuid, and those in which the metals were ex- 
posed freely to air, and covered by an indefinitely thin film of 
water constantly renewed, or, in technical language, to ‘‘ wet 
and dry. In each of these conditions experiments have been 
made on the protecied metal, in presence of zinc in a massive 
form in simple contact, through the intervention of the solvent 
or fluid in which both were immersed, and also when the pro- 
tected metal has had voltaic contact established with the zinc by 
