PAPERS ON PHYSICS 293 
No. 1, No. 2 and No. 3 of the previous experiment were tested. 
Observations were taken at intervals of 15 minutes for the 
first 4 hours, of 30 minutes for the second four hours, and of 
1 hour for the last four hours. 
As a check upon the rate of cooling after the highest tem- 
perature had been reached, the normal cooling curve was ob- 
tained for coal No. 3. This was obtained by again heating the 
60 pounds of water to 180° F., then removing the boiler from 
the stove and setting it upon a wooden bench. The rate of 
cooling was then determined. 
CONCLUSIONS 
1. I conclude that the rate at which these different Illinois 
coals burn varies greatly under unvarying conditions. Prob- 
ably this variation is largely due to the bond, or bonds, which 
exist between the fuel elements in their composition. It is 
probable that the nature of this bond, or these bonds, deter- 
mines largely the rate at which the volatile portion of bitum- 
inous coal is liberated when the coal is heated. 
2. It does not appear from these tests that there is any evi- 
cent connection between relative proportions of fixed carbon 
and volatile matter on the one hand and the rate of combustion 
on the other hand, In these tests the slowest burning coal had 
the highest percentage of volatile matter and the lowest per- 
centage of fixed carbon, while the coal having the next highest 
percentage of volatile matter and the next lowest percentage of 
fixed carbon proved to be the most rapid burning coal. These 
facts confirm me in the belief that the rate of combustion of 
these coals is chiefly determined by the bonds which tie the 
fuel elements together and possibly also which tie these ele- 
ments to the other elements composing the coal. 
3. I further conclude that the use or uses to which the coal 
is to be put should be clearly recognized in the purchase of 
coal. It is my belief that the rate at which a coal distills when 
heated is a larger factor in determining the rate at which it 
burns under ordinary conditions of domestic use than is the 
relative proportions of volatile matter and fixed carbon. If 
the conditions under which the coal is to be burned permit of 
complete combustion, or as nearly complete combustion as is 
