ABSORPTION AND PENETRATION OF COAL TAR AND CREOSOTE. 11 
probably due, as noted in the case of the penetrance tests, to the 
character of the carbon mat which formed when this tar was used. 
Blocks treated with the mixtures containing varying amounts of 
free carbon were found to be fairly well penetrated. By using a 
sufficient treating period and pressure, and a temperature of 200° F., 
it was possible to obtain 16 pounds absorption per cubic foot hi all 
of the treatments on paving blocks. 
The experiments taken in conjunction with the penetrance tests, 
indicate that three factors influenced absorption and penetration, 
namely, (1) the composition or per cent of bitumens contained in 
the tar, (2) the amount of free carbon contained in the mixture, 
and (3) the condition of the carbon or size of the free carbon agglom- 
erations. 
INFLUENCE OF COMPOSITION OF THE TARS. 
The influence of the composition or per cent of bitumens contained 
in the tar is shown by the fact that the addition of high-carbon tars to 
creosote retarded penetration more than similar additions of low- 
carbon tars, even though the carbon had been removed. The amount 
of carbon in tar depends to a considerable extent upon the tempera- 
ture at which it is produced, the higher temperatures producing tars 
with greater carbon content. It is concluded that tars produced at 
high temperatures may have a greater effect in retarding penetration 
than those produced at lower temperatures, whether or not the carbon 
is removed. Experience has shown that tars having a high carbon 
content also usually have a high bitumen content. 
The condition of the free carbon apparently had a greater influence 
on absorption and penetration than the amount contained in the 
mixture. Since the free-carbon agglomerations were largest in the 
tar containing the highest percentage of free carbon (tar No. 3), 
this probably accounts for the relatively slight retarding influence of 
the free carbon hi this tar, on absorption and penetration. In the 
mixtures containing smaller amounts of free carbon, the free carbon 
particles would come in closer contact with the vessels in the wood, 
thereby decreasing the absorption and penetration. 
PENETRATION OF FREE CARBON INTO WOOD. 
In order to determine whether any of the free carbon had pene- 
trated the wood, sections were taken of specimens of heart longleaf 
pine treated with mixtures of equal parts of creosote and tars Nos. 1, 2, 
and 3, containing the normal amounts of free carbon. These sections 
were taken just below the surface through which the preservative 
entered. A microscopic examination was made at a magnification 
of 400 diameters but no free carbon could be observed in the wood 
'cells. Some of the preservative was squeezed out of the wood sections 
in small globules and examined under the microscope. In this case 
