10 
BULLETIN 607, U. S. DEPARTMENT OF AGRICULTURE. 
pressure were varied so as to obtain approximately 16 pounds absorp- 
tion per cubic foot. 
Figure 5 indicates that when the amount of free carbon in the 
mixtures was increased, the difficulty of securing a given absorption 
IMPREGNATION TESTS. 
(Data in Tables 11, 12, and 13.) 
^0 
2"- 18 
oS |4 
s~ 
TAR M 
r TAR 
3 
VT 
AR 
NO- 
>oa 12 
10 
1000 
900 
800 
^TAf 
700 
** i 
i 
600 
s 
^-TAR 
M0. 2 
» 
K 
X 500, 
u 
i** 
D 
a 400 
TAI 
\ N0.I 
BL 
300 
200 
100 
000 
1 
3 4 
5 6 7 8 9 10 It 12 13 14 15 16 17 18 19 20 
PERCENT FREE CARBON IN MIXTURE 
Fio. 5.— Variation in time of treatment and pressure necessary to secure a given absorption in paving blocks 
using mixtures of equal parts creosote No. 4 and tars having different amounts of free carbon. 
was increased. As in the penetrance tests, however, the effect of free 
carbon was variable in the mixtures containing tar No. 3. The 
retarding effect of free carbon apparently decreased when the amount 
in the mixtures was increased beyond a certain point. This was 
