TRANSACTIONS OF THE SECTIONS. 
37 
clean and free from corrosion; the zinc has of course been corroded. 
In a comparative experiment, in which a similar piece of tin plate 
was nailed to the same piece of wood, and exposed during the same 
period to the same quantity of sea-water, without the zinc, the edges 
on two sides of the tin plate were quite soft from the corrosion, 
which had extended to about -£ th of an inch. These experiments 
seem worthy of being repeated and extended. 
The present demand for tin plate is very great; should these 
statements be confirmed, a vast increase in its consumption might 
be anticipated. The opinion may be entertained that it is practi¬ 
cable to substitute double tin plate for sheet copper in covering 
the bottoms of ships, &c., using zinc in small proportion as a pro¬ 
tector. Such applications would probably occasion a saving of 
nearly three fourths of the present expense of copper sheathing. 
It also seems deserving of inquiry whether tin plate vessels, pro¬ 
tected by zinc, may not be advantageously substituted for copper 
vessels in many of our arts and manufactures, and even in domestic 
oeconomy. Although it might be presumed from Sir H. Davy’s ex¬ 
periments and observations* that zinc would protect tin plate from 
corrosion in sea-water, the author is not aware that any direct ex¬ 
periments on the subject have been published. Sir H. Davy briefly 
refers to some obvious practical applications of his researches to the 
preservation of finely divided astronomical instruments of steel by 
iron or zinc ; and that Mr. Pepys had taken advantage of this last 
circumstance in inclosing fine cutting-instruments in handles or 
cases lined with zinc. The author has not heard whether such ap¬ 
plications have succeeded, but he has made a number of experiments 
with a view to protect brass, iron, copper, &c. from tarnish and 
corrosion in the atmosphere by means of zinc ; the results obtained, 
however, lead to the conclusion that contact with zinc will not protect 
those metals in the atmosphere, the electricity thus produced, with¬ 
out the intervention of a fluid, being apparently too feeble to coun¬ 
teract the chemical action of air and moisture on the surfaces of these 
metals. 
On the comparative value of Irish and Virginian Tobacco. By 
Edmund Davy, F.R.S., M.R.I.A., <^c., Professor of Chemistry 
to the Royal Dublin Society . 
In the year 1829-30 the cultivation of tobacco in Ireland excited 
much attention among agriculturists, and several hundred acres of 
it were raised in different counties ; in consequence, the attention of 
the Royal Dublin Society was directed to the subject, and the author 
was requested by a select committee of that body to institute ex¬ 
periments on tobacco with a view to determine some questions of a 
* Phil. Trans, vol. cxiv. for 1824; or Phil. Mag., first series, vol. lxiv. 
pp. 30, 233 ; vol. lxv. 203. 
