PAPERS ON PHYSICS 287 
Not only is the pump one of high speed, but it is also one 
giving an extremely high vacuum. The limit to which the 
pressure in the evacuated vessel tends is zero, or, more strictly, 
to the vapor pressure of glass, or of aluminum, i.e., to that 
of the material of which the electrodes are made. Obviously 
if no trap for freezing out the mercury vapor is attached, the 
limit of the pressure attainable is that of mercury. When 
made of pyrex glass the pump is durable. 
THE RATE OF COMBUSTION OF SOME ILLINOIS 
COALS 
Frep D. BarsBer, ILLINOIS STATE NORMAL UNIVERSITY 
The importance of studying the character and relative heat- 
ing values of bituminous coals is every where recognized. Prox- 
imate and ultimate analyses of all important outputs of Illinois 
have been made, and their calorific values determined, by the 
State Geological Survey; these are easily accessible. The 
necessity of distinguishing between total volatile matter and 
combustible volatile matter is also well recognized. Many sug- 
gestions have been made regarding the classification of coals 
on the basis of facts revealed by proximate analysis. Similarly, 
generally accepted formulae have been worked out for de- 
termining the fuel units in coals from the facts obtained from 
analysis, thus saving the labor of determining directly their 
calorimetric values. 
Notwithstanding the progress made in the study of coals, 
especially bituminous coals, and the vast amount of data col- 
lected, the statement by H. Foster Bain, Director of the State 
Geological Survey, in his letter transmitting Bulletin No, 3 on 
The Composition and Character of Illinois Coals to Governor 
Deneen, in 1906, is probably as true today as it was the day 
it was written. He said, “Much more information is needed 
regarding the real nature of coal and the state of combination 
of its elements. Professor Parr’s earlier results had shown 
that coals of the same composition, as measured by ultimate 
and proximate analyses, might differ greatly in character and 
adaptability because of the different sorts of bond existing be- 
tween the carbon, hydrogen and other elements. It is import- 
ant to know what these combinations are and to devise a 
