205 
On Preventing the Decay of Wood. 
brushed off what was loose, and over that which adhered 
I applied a coat of common lead- coloured paint, and a 
few days after, a second. The whole became a firm and 
solid crust ; after which the shoots were put into their 
places, and being examined many years afterward, ap 
peared perfectly sound. Any other colour would proba- 
bly have succeeded equally well with that which I em= 
ployed. I do not think that lamp black, which is a pure 
species of charcoal, would have answered the purpose 
of forming a thick defensive covering so well as the 
grosser charcoal which I used. But whatever sort of 
charcoal is employed, it ought either to be fresh made, 
or heated again in close vessels, so as to expel the water 
which it greedily attracts from the air. 
To all compositions formed from drying vegetable oils 
there is this objection; that however well they may an- 
swer the end proposed, they are too dear for that great 
consumption, which is usually required for outside work. 
For this and other reasons, various other substances have 
been employed for the same purpose. 
Of these the most common is pitch, which is well known 
to be the resinous matter melted by heat out of the pine 
tribe of trees in form of tar, and afterwards hardened by 
evaporation. It is applied hot, and when cold, makes a 
moderately hard varnish. It does not however appear, 
in fact, to answer the purpose so well as might have been 
expected. The sun at first melts it, so that it runs off in 
drops, or adheres to every thing which touches it; and 
the united influence of air and water seems to make it 
brittle and powdery like resin. Experience, therefore, 
shows it to be of little value. Neither is it probable that 
its powers would be much improved by admixture with 
charcoal, sand, or other similar substances. Many mem- 
bers of this Society may recollect its application twenty 
years ago on the red- deal shingled roofs of part of our 
Vol. ii. p p 
