O EXPERIMENTAL WORK IN FUEL TESTING. 
DETERMINATION OF ERRORS DUE TO ABRASION OF PEBBLES USED IN 
BALL MILL. 
• During the Louisiana Purchase Exposition the final pulverization of 
the samples was done on a bucking board. In the work done during 
1905 the final grinding of the sample was done in closed jars in the 
ball mill, quartz pebbles being used. 
In order to determine whether there was danger of materially increas- 
ing the ash contents of the samples from chipping and abrasion of the 
pebbles used in grinding, these pebbles were carefully weighed at inter 
vals and the amount of loss determined. The weight of the sample 
ground each time in the ball mill is approximately 500 grams, and the 
abrasion of the pebbles (calculated as percentage of the weight of the 
samples ground) was also carefully determined. The results on three 
weighed lots of pebbles are as follows: 
Losses of weight in three lots of quartz pebbles before and after grinding in the ball mill. 
Lot. 
No. 1 (250 samples ground) 
No. 2 (230 samples ground) 
No. 3 (245 samples ground) 
Total 
weight of 
samples 
ground. 
Grams. 
125,000 
115,000 
322,500 
Weight of pebbles. 
Before 
grinding 
Grains. 
4,118.6 
3, 502. 9 
4,273.2 
After 
grinding 
Grams. 
4,113 1 
3, 499. 4 
4, 268. 3 
Loss by abrasion. 
Actual. 
Grams. 
5.5 
3.5 
4.9 
Ratio to 
coal 
ground. 
Per cent. 
0.004 
.003 
.004 
These results show that as far as the fine pulverization of the coal is 
concerned, there is little or no danger of increasing the ash content 
of the sample appreciably by using quartz pebbles as the grinding 
medium. The pebbles used show no tendency to chip, as the loss 
between weighings taken before and after 25 to 50 samples were 
ground in no case amounted to over 1 gram, the abrasion being in all 
cases approximately proportional to the weight of samples ground. 
COMPARISON OF RESULTS OF AIR DRYING IN THE SPECIAL OVEN AND 
BY EXPOSURE. 
The samples as received at the laboratory were dried to approxi- 
mately an air-dry condition, before their final pulverization, in an 
oven especially designed for this purpose. To find out how nearly this 
method of drying approximates air drying under ordinary conditions, 
the following tests were made: 
The loss in weight of samples of different coals allowed to air dry 
under observed conditions of temperature and humidity was deter- 
mined by allowing portions of the coarse samples spread on trays to 
remain exposed to the air of the laboratory for periods of time rang- 
ing from seven to twenty days. The samples were weighed from time 
