BULLETIN 607, U. S. DEPARTMENT OF AGRICULTURE. 
Per cent of total mixture by volume. 
Creosote 
No. 4. 
Tar. 
100 
75 
50 
25 


25 
50 
75 
100 
In order to obtain data on the effect of free carbon on absorption 
and penetration, it was necessary to vary the amount of free carbon 
in each of the three tars. This was accomplished by combining the 
proper proportions of normal * tar with tar of the same origin from 
which the free carbon had been removed. These mixtures were made 
by weight. The tars containing the varying percentages of free car- 
bon were then mixed with creosote by volume in the proportion of 
50 per cent tar and 50 per cent creosote. 
CHARACTER OF FREE CARBON. 
The free carbon was extracted with the least difficulty from the tar 
containing the highest percentage of free carbon (tar No. 3) and with 
the most difficulty from the tar containing the lowest percentage of 
free carbon (tar No. 1). A greater number of nitrations were re- 
quired with tar No. 1, and a mat of free carbon of given thickness 
was more inclined to be impervious to additional tar than in the case 
of the other tars. The carbon mat formed from the tar having the 
largest amount of free carbon appeared to be the most pervious. 
Tars Nos. 1, 2, and 3 were examined under the microscope at a mag- 
nification of 400 diameters. Plates I to III, inclusive, are photomi- 
crographs of thin films of mixtures of equal parts by volume of creo- 
sote and each of the three normal tars. It is evident that the free 
carbon agglomerations are largest in the tar containing the highest 
percentage of free carbon. 
PRESERVATIVES USED FOR THE TESTS ON THE EFFECT OF VARYING TIME, PRESSURE, 
AND TEMPERATURE. 
By-product coke-oven tars Nos. 1, 4, and 5, and coal-tar creosotes 
Nos. 4 and 6 were used in the experiments described on page 12. The 
proportion of tar and creosote in the mixtures and the temperatures, 
pressures, durations of treatment, and the results obtained are given 
in figures 18 to 27 and in tables 15 to 21 in the Appendix. 
1 By normal tar is meant tar containing its original free carbon. 
