8 THE PUKCHASE OF COAL UNDER SPECIFICATIONS. 
An air supply slightly in excess of the theoretical amount required 
for complete combustion. 
A sufficiently high temperature to ignite the gases which are driven 
off from the fuel. 
A complete mixture of these gases with the air supplied before 
they reach a cooling surface, such as the shell or tubes of a boiler. 
Some of the factors which may influence the commercial results 
obtained in a boiler are the cost of the coal, as determined by price 
and heating value; care in firing; design of the furnace and boiler 
setting, size of grate, etc. ; formation of excessive amounts of clinker 
and ash; draft available; size of the coal (uniformity of size is 
desirable). 
The value of a coal is indicated by the number of heat units it 
contains. This heating value is expressed in terms of British thermal 
units a (abbreviated B. t. u.) per pound of coal, and is determined by 
means of a special apparatus called a calorimeter. 
When coal is mined it contains moisture to a greater or less extent. 
It is exposed to the air in shipment and may either dry out or be 
drenched by rain. The moisture in the coal delivered is worthless to 
the purchaser, and really costs him a considerable amount in freight 
and cartage, and in the loss of the heat' absorbed during its evapora- 
tion in the furnace. If all coal had the same proportion of moisture, 
or if the moisture in coal delivered by a given dealer was constant in 
amount, the purchaser's problem, so far as this factor is concerned, 
would be simplified. Under present conditions the moisture is an 
important element in the valuation of a ton of coal. It is evidently 
necessary to consider the coal just as it is received in order to 
determine its value to the consumer, but chemical reports should be 
made on the basis of both the " dry coal " and the " coal as received." 
The dry-coal basis is convenient for comparing several coals in regard 
to the relation of each element to the others ; this is important because 
the moisture in the same coal varies from day to day. The dry-coal 
basis is also convenient for comparing the performance of boilers 
when burning the same or similar coals. Of several coals having a 
similar composition, the one which has the least moisture and the 
least ash will generate the most steam when burned under a boiler. 
Ash is made up of earthy matter and other impurities which will 
not burn. In commercial coals its proportion may range from 4 to 
25 per cent. Coals containing small percentages of ash are most 
valuable, not only because of their correspondingly higher heating ca- 
pacity, but because there is less resistance to the free and uniform 
distribution of air through the bed of coal. The labor and cost of 
managing the fires and of handling the ashes are also correspondingly 
° The British thermal unit is the amount of heat required to raise the temperature of 
1 pound of water 1° Fahrenheit. 
