ON THE ACTION OF AIR AND WATER UPON IRON. 243 
by the microscope ; and thus, when laid on an oxidable metal, 
corrosion slowly takes place through these, or, as workmen 
say, “it rusts under the paint.” 
198. In the absorption of oxygen, which takes place when 
an oil capable of being rendered drying, such as linseed or nut 
oil, is exposed to the action of the atmosphere or of oxygen 
gas, the volume of carbonic acid formed is by no means equi- 
valent to that of the oxygen absorbed. Hence it is obvious 
that the drying of oily paints is effected by a slow but real 
combustion of part of the hydrogen of the oil forming water, 
and by the partial acidification of the vegetable mucilage. This 
process is greatly accelerated by the presence of various sub- 
stances, but particularly carbon in a solid state. It is well 
known that lamp-black and oils mixed together in proper pro- 
portions absorb oxygen so fast as to produce spontaneous com- 
bustion at high temperatures, yet it is the slow combustion of 
the hydrogen of the oil rendered more active by presence of 
the carbon, to which the exaltation of temperature sufficient 
to produce ignition was at first due. This combustion is already 
in part performed in the oils rendered drying by litharge, &c., 
and still more in the burnt oil used for the ink of copper-plate 
and letter-press printers, and called by them “the varnish.”’ 
In fine, in every combustible compound of carbon and hydro- 
gen, combustion, whether slow or defectively supplied with 
oxygen, seizes on the hydrogen first, and lastly on the car- 
bon, and this leads, in the case of oil paints, to a succession 
of changes of constitution, by which at length the original 
solid material of the paint alone remains in feeble combination 
with a little decomposing resin. 
199. The direction, then, in which we are able to look for 
_ improvement in the preservative power and durability of our 
_ paints, is in choosing from amongst the known groups of 
_ organic substances, those which have greater stability than 
the fat or fixed oils, and which, in place of being acid or 
_ haloid, are basic or neutral. Amorigst the many substances of 
_ this class which occur, few seem better fitted to form a sub- 
_ Stitute for the fat oils in paints than the heavy oily matter 
obtained by the distillation of resin, to which M. Fremy gave 
the name of ‘ resinein*’; it has the composition (C,, H,, O). 
Now two atoms of resin = 2 (C,, H, O); hence this oil is 
a fluid resin deprived of an atom of water =(H O). It isa 
heavy transparent oil, destitute of taste or smell, insoluble in 
Water and alcohol, not acted on by caustic alkalies, has a high 
boiling-point = 480° Fahr., and reduces litharge, when boiled 
* Annales du Pharm. xv. 282. 
R 2 
