[DOWLING] THE FORMATION OF COAL 35 
These suppositions may serve to suggest reasons why an anthracite 
may be low in ash while softer coals of the same age may have much 
more. They show also that the escape of hydrocarbons may not raise 
the carbon percentage as rapidly as has been generally supposed. 
There is here also a suggestion that outcrops of coal seams may by the 
loss of gases be high in ash and not necessarily very high in carbon. 
The general assumption that the carbon percentage shows the 
amount of alteration may not always be true. It may show the degree 
but is not always an index of the amount lost. 
It has been pointed out that the hydrogen percentage has a direct 
bearing on the character of the coal. During the final process there are 
many chances that the hydrogen may increase or diminish. With an 
alteration depending on the formation of water the hydrogen may fall 
below the mean; but the concurrent formation of carbonic acid raises it 
rapidly. With the escape of combustible gases it may fall to zero and at 
almost any percentage of carbon obtained. The chances of producing 
fatty coals seem greater for the process requiring confined pressure. 
SUMMARY. 
Variations in coal may be due to — 
(1).—Original composition.—The softer varieties of vegetation 
having an initial high hydrogen percentage are rapidly dessiccated and 
prepared for fermentation and subsequent change. 
(2).—Duration and character of decay.—The initial loss of carbon 
dioxide may be prolonged by fermentation, thereby raising the hydrogen 
to the critical point at which it forms an unstable hydiocarbon com- 
pound. The character of the product thus depends in great measure on 
the stage reached in the process. 
(3).—Pressure.—Restrains the loss of carbon and promotes the 
formation of water and carbonic acid. This produces an alteration 
without raising the ash unduly or allowing the hydrogen to be lowered 
very much. The process is hastened by heat. 
Heat causes the formation of hydrocarbons gases and raises the ash 
by the great loss of material. The loss of hydrogen also tends toward 
the production of a dry or lean coal. 
Sec. IV., 1910. 3. 
