The gas-coke in the table is that sold by the local gas company. The amount of moisture in this case is relatively high because of the fact that the coke is stored in a yard exposed to the weather, where it absorbs all precipitated moisture. A less amount of moisture would give a higher value for the same fuel.

Kind of coal Price
per
ton		 Per cent.,
water		 B.t.u.
per 100
pounds		 B.t.u. to
evaporate
water		 B.t.u.
per 100
÷ cost per
100 pounds		Heat per $1
Bituminous
Pennsylvania		 $7.50 2.44 1,340,000 - 3,439 = 1,336,565
————
$0.375		 = 3,564,000 B.t.u.
Semi-
bituminous
West Virginia 		$9.00 3.06 1,420,000 - 4,315 = 1,415,685
————
$0.45		 = 3,145,000 B.t.u.
Gas-coke $7.00 10.00 1,117,900 - 16,888 = 1,101,012
————
$0.35		= 3,145,000 B.t.u.
North
Dakota
lignite		 $4.50 35.90 668,000 - 50,728 = 607,282
———
$0.225		 = 2,703,000 B.t.u.
Bituminous
Illinois		 $7.50 10.31 1,032,000 - 14,398 = 1,017,602
————
$0.375		 = 2,980,000 B.t.u.
Bituminous
Iowa		 $7.50 13.10 1,012,000 - 18,471 = 994,529
———
$0.375		 = 2,652,000 B.t.u.
Hard coal
Pennsylvania		 $10.50 3.05 1,230,000 - 4,195 = 1,225,905
————
$0.525		 = 2,335,000 B.t.u.

Semi-bituminous coal commands considerable favor as a house-heating fuel, because of the fact that it burns with much less smoke than bituminous coal. In available heat it is considerably above the Western bituminous coal and it sells at a price $1.50 higher per ton. The reason for the difference in price is not so much on account of its heating value, as because of relatively small amount of smoke produced in combustion. Other coals capable of producing more heat are sold at less price because of smoke and soot produced in burning.

Hard coal at $10.50 is the most expensive coal of all. The ratio of available heat units per $1 for hard coal, as compared with the best soft coal, is as 23 is to 35. This means that at the stated prices those who burn hard coal pay the additional price, because of the physical properties it possesses.

In constructing the above table, 100 pounds of coal was taken as a unit of comparison. The price per ton is that given in the table of local prices. The per cent. of moisture and the B.t.u. per pound of fuel was taken from table on [page 192].

In explaining the method by which the different items were obtained, it will be necessary to discuss briefly the condition of combustion and the heat losses that take place when fuel is burned.

The moisture in the fuel is the undesirable part, because it requires a large amount of heat to dispose of it. It is looked upon as so much water, that must be raised in temperature from that in which it is taken from the coal bin to the temperature and condition of vapor in which it passes into the chimney. When the fuel enters the furnace the water is heated to the boiling point. In changing temperature it absorbs 1 B.t.u. for each pound of water, through each degree of change. Suppose that, as in the case of Pennsylvania bituminous coal which contains 2.44 pounds of water to each 100 pounds of coal, the coal entering the furnace was at 50°F. To raise its temperature to the boiling point (212°F.) required a change of 162°. The 2.44 pounds of water raised this amount

162 × 2.44 = 395.28 B.t.u.

To change the 2.44 pounds of water, into steam at the atmospheric pressure requires 969.7 B.t.u. (heat of vaporization), practically 970 B.t.u. per pound of water. The heat required to vaporize 2.44 pounds of water is