BULLETIN 607, U. S. DEPARTMENT. OF AGBICULTUEE. 

 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 l 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 OP 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. 



