302 On Preventing the Decay of Wood . 
for ten times that period. Let us see then if we cannot* 
by the exclusion of moisture and air* find means of virtu- 
ally placing our timber in a case of glass or amber. 
With this view* various expedients have been employ- 
ed* of which the most common is covering the surface with 
paint; which is oil mixed with some substance capable 
of giving it the colour which we desire. It is well 
known* that several of the oils* as those of linseed* hemp- 
seed* &c. become dry when thinly spread on any hard 
substance. The drying quality is much assisted by their 
being previously boiled with certain metallic oxides* 
more especially that of lead* litharge. The crust so 
formed is with difficulty penetrated by moisture or air. 
For this purpose drying oil is spread on silk or linen* in 
the manufacture of umbrellas; and will tolerably well 
succeed in confining hydrogen gas* or inflammable air* in 
the construction of air-balloons. Hence we see the mode 
in which the application of paint on wood serves to de- 
fend it against the causes of destruction. 
When paint is employed within doors* it is customary 
to add to the oil* beside the colouring matter* some essen- 
tial oil of turpentine* which not only makes it dry more 
readily* but* by giving it greater tenuity* causes it to flow 
more freely from the brush* and therefore to go farther in 
the work. F or the same purposes I observe it forms a 
part of the paint used on wood and iron work in the open 
air; but* as it appears to me* most improperly: For I 
have remarked that on rubbing wood painted white* and 
long exposed to the weather* the white lead has come oflf 
in a dry powder like whiting; as if the vehicle which 
glued it to the wood had been decomposed and lost* leav- 
ing only the pigment behind : And I have been much in- 
clined to suspect* that this has arisen from the oil having 
been too much opened, as the workmen call it* or having 
its thickness and tenacity too much diminished by a m 
