80 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY, 
steadings, greenhouses and buildings of that class, as well as 
flooring, joists, and roofs of dwelling-houses, the process may 
be adopted with very great advantage. It is simple and in- 
expensive; sulphate of copper being a cheap product, costing 
usually from 12s. to 14s. per ewt., which with 2 lbs. to 20 gallons 
of water, and half a gallon to the cubic foot, makes a cheap but 
efficient preparation, and the mode of applying it simple. The 
dry seasoned wood is placed in an open tank, covered over with 
the solution, and left in the tank till thoroughly saturated. It is 
then taken out and allowed to dry under cover. When dry it is 
ready for use. 
The term “creosote” is not quite accurate. Strictly speaking, 
creosote is the product of the distillation of wood, and has never 
been used for the preservation of timber. The “creosote” of 
commerce is distilled from coal-tar, which does not contain any of 
the true creosote. 
When the coal-tar produced in the manufacture of illuminating 
gas is distilled, it is exposed in an iron still to a gradually 
increasing temperature, which ranges during the process from 
180° to 750° or 800° Fahrenheit. The tar yields three main 
groups of products, viz.,—oils lighter than water, or crude 
naphthas; oils heavier than water, called dead or heavy oils, 
or “oil of tar”; and lastly pitch, the latter being the residue 
left in the still. The products are obtained very nearly in the 
order of their specific gravities, the more volatile coming over first, 
and being followed at intervals by heavier and heavier portions 
as the heat increases. Each portion can, if necessary, be collected 
separately as it comes over. 
By further treatment, the first two groups can be resolved into 
their constituents. The light oils yield chiefly benzol, naphtha, 
and carbolie acid, but they are too volatile to be useful in the 
preservation of timber. The heavy oils, which in their crude form 
constitute the creosote of commerce, produce carbolic acid and 
other tar acids, the solid substances naphthaline and anthracene, 
and the heavy lubricating oils. 
There has been a guod deal of controversy on the question as to 
which constituent the preserving power of creosote is chiefly due. 
For some time after the antiseptic powers of carbolic acid were 
fully recognised, it was considered that the efficiency of creosote 
depended greatly upon the percentage of tar acids which it con- 
tained, and that the lighter portions of the dead oils were the best, 
