ON THE ACTION OF AIR AND WATER UPON IRON, 239 
salt, white-lead paint perishes at once, the white-lead being 
probably converted into sulphuret by the action of the nascent 
hydro-sulphuric acid ; yet white lead forms the great staple 
base for all the paints generally used for exposed iron works. 
Caoutchouc varnish appears to be the best covering in hot water, 
and generally in all the others asphaltum varnish. But boiled 
coal tar laid on, the iron being hot, has decided advantages over 
every other, the reasons of which we shall presently see. It is 
sufficiently obvious, however, that nothing very important in 
the way of protection is to be hoped for from any one of the 
coverings tried, at least when used alone. 
194. Since these experiments were commenced, my atten- 
tion has been drawn to the results of an analogous series obtained 
by Mr. James Princep of Calcutta, and contained in the Journal 
of the Asiatic Society of Bengal, which appear worthy of notice. 
The proposed extensive employment of iron steam-boats on the 
Ganges, and the rapid corrosion to which iron is subject in a 
hot climate, induced the local government to cause these expe- 
riments to be made, to endeavour to procure a varnish that 
should preserve the surface of the metal. Mr. Prinsep took two 
sets of six wrought iron plates, each 3 feet by 2 feet, upon which 
various paints and varnishes were applied. One set was just 
completely submerged, and the other half-immersed, one half of 
each plate being in the air and the other half in the water of the 
canal at Calcutta, near the Chitpur lock-gates, where it is only 
slightly salt. After three months’ exposure the two sets were 
taken up and examined, and the following table contains the re- 
sults, which it is to be regretted have not the numerical pre- 
cision that would have been obtained by weighing the plates 
before and after immersion. 
Experiments 
