14 BULLETIN 607, U. S. DEPARTMENT OF AGRICULTURE. 
servative. Two series of tests were made with the temperature and 
pressure of the preservative constant and the time of treatment 
varied to obtain the same absorption with the mixture as was obtained 
with the creosote unmixed with tar.' Another series of tests was run, 
in which the pressure of the preservative was varied but the tempera- 
ture and the time of treatment kept constant, and a comparison made 
of the pressures required to obtain the same absorption with various 
tar mixtures as was obtained with creosote unmixed with tar. A 
further series of tests was made in which the time of treatment, 
pressure of the preservative, and absorption were constant, but the 
temperature was varied. For comparison a test was made of the 
absorptions and penetrations obtained using the same time of treat- 
ment, temperature, and pressure of preservative for both the creosote 
mimixed with tar and for the tar mixtures. 
The system of matching the penetrance specimens used in the 
experiments described on pages 4 to 20 was the same as that described 
on page 2, except that in one test four specimens were used and in 
other tests three specimens were used instead of two, as in the 
previous experiments. All of the blocks used in the impregnation 
tests were cut from the same stick and are, therefore, considered 
matched with each other. 
PENETRANCE TESTS WITH COAL-TAR CREOSOTE NO. 6 AND BY-PRODUCT COKE-OVEN 
TAR NO. 4. 
Time of treatment, temperature of preservative, and pressure con- 
stant. — (Table 15 and fig. 7.) 
The first series of tests was made to study the relative absorptions 
and penetrations obtained with the creosote unmixed with tar; with 
mixtures of 80 per cent creosote and 20 per cent tar; and with 66 § 
per cent creosote and 33 J per cent tar. The pressure, the time of 
treatment, and the temperature of the preservative were the same 
for every preservative. These tests were made for comparison with 
those on the same mixtures where the pressure, time, etc., were 
varied. Details of the treatment are given in Table 15. 
The average absorption and the average longitudinal penetration 
obtained with the 80 per cent creosote and 20 per cent tar mixture 
were each about 73 per cent of that obtained with the creosote. In 
the case of the 66§ per cent creosote and 33 J per cent tar mixture 
the absorption was about 66 per cent and the average longitudinal 
penetration about 69 per cent of that secured with creosote. 
