264 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
These consist of the volatile hydrocarbons and of the nonvolatile or 
fixed carbon. This relation is expressed by the fuel ratio, a quantity 
obtained by dividing the percentage of fixed carbon by the percentage 
of the volatile combustible constituents of the coal. In general the 
fuel value or heating power increases with the increase of the fuel 
ratio, since more heat is developed in the combustion of carbon than 
of the hydrocarbon compounds. This increase in fuel value, how- 
ever, continues only to a certain point, beyond which the difficulty of 
effecting combustion more than makes up for the greater amount of 
heat evolved. Thus the graphitic anthracite of the Rhode Island 
field can not properly be regarded as a fuel, since the percentage of 
volatile constituents is so small that these have to be supplied by. the 
addition of another coal before it will burn. 
In addition to its fuel constituents, a coal contains others which are 
nonessential. The most important of these are water and ash. The 
former not only replaces an equal weight of combustible matter but 
also absorbs heat in its volatilization. An excessive amount of water, 
therefore, detracts seriously from the fuel value of a coal. Its pres- 
ence is further detrimental in causing the coal to break up into fine 
particles as it dries out. The amount of water generally varies 
inversely as the fuel ratio, being less than 1 per cent in some anthra- 
cites and from 15 to 25 per cent in lignites. 
The ash simply occupies the place of combustible matter and is in 
general purely negative in its influence on the fuel. When very 
abundant it may seriously retard combustion, and Avhen it contains 
easily fusible constituents it may become a positive detriment by 
forming clinker on the grate bars. Sulphur is detrimental in a steam- 
ing fuel chiefly by reason of the corrosive effect that its products of 
combustion exert on iron surfaces with which they come in contact. 
For most metallurgical purposes it is essential that the coal should 
be relatively free from certain injurious constituents, such as sulphur 
and phosphorus. 
The amounts of water and ash which a coal contains are not shown 
by its fuel ratio, and hence this does not serve to indicate its fuel value 
so much as its adaptability for specific purposes. Thus it is evident 
that for gas-producing purposes a coal should be chosen having a 
large proportion of volatile constituents; in other words, a Ioav fuel 
ratio. 
The coking quality of a coal depends on conditions which are in 
a measure independent of its chemical composition, although coking 
coals do not have a very wide range in fuel ratios, which generally 
fall between 1. 20 and 2. 50. By no means all coals will coke, however, 
whose ratios fall between these limits. 
The coal of the Carboniferous fields considered as a whole show a 
decrease in their fuel ratios from east to west. In the Rhode Island 
field the coal has suffered so high a degree of metamorphism that it 
