ON LIQUID FUEL. 
101 
Relative Calorific Power. 
Pound. Heat units. 
Carbon . . . 1 . . . 14,500 . . . 1-00 
Hydrogen . . 1 . . . G2,032 . . . 4-28 
The heat unit here referred to is the quantity of heat which raises the temperature of 
one pound of water one degree F. (from 40° to 41°). Therefore, the numbers given in 
the table represent the quantities of water capable of being heated one degree F. by the 
conversion of one pound of carbon into carbonic acid gas, or of one pound of hydrogen 
into water. As there are, in the Fahrenheit thermometric scale, 180 degrees between 
the freezing-point and boiling-point of water, those numbers divided by 180 give the 
corresponding quantity of water capable of being heated from 32° to 212° F. Again, 
the quantity of heat required to convert a pound of water at 212°F. into steam of the same 
temperature, is nearly 5* (more exactly 5'37) times as much as that requisite to heat 
a pound of water from the freezing-point to the boiling-point ; therefore, the quantities 
of steam capable of being produced from water at 212° F. by the total heat generated 
in the combustion of a pound of carbon or of hydrogen, are of course ascertainable by 
dividing the number of pounds heated from 32° to 212° F. by 5'37. The several quan¬ 
tities are given in the following table :— 
Quantities of Water. 
r ~ 
Heated. 
Or converted into 
steam. 
By the heat generated 
in combustion of. 
From 
From 
From 
40° to 41° F. 
32° to 212° F. 
water at 212° F. 
lbs. 
lbs. 
lbs. 
lbs. 
14,500 
80‘55 
15* 
1 carbon. 
62,032 
, 
344-62 
ca 
1 hydrogen. 
These quantities of 15 pounds and 64 "2 pounds of water convertible into steam by 
the total heat generated in the combustion of a pound of carbon or of hydrogen, repre¬ 
sent what is termed the “ theoretical evaporative powers ” of those substances. By the 
term theoretical, however, it is not to be understood that these values are in any degree 
imaginary or assumed; they represent actual facts, which have been established as the 
results of positive observation, and they are theoretical in reference to the practical ap¬ 
plication of fuel only in this sense, that these results are not realized in ordinary practice. 
The reason of this is not the existence of any uncertainty that the total quantities of heat 
generated by burning a pound of carbon or a pound of hydrogen are respectively capable 
of converting 15 pounds and 64’2 pounds of water at 212° F. into steam, but it is simply 
the fact that, under ordinary circumstances, only a portion of the total heat generated in 
either case is ever available for the production of steam. The statement of the theore¬ 
tical evaporative power of fuel, or of carbon and hydrogen as constituents of fuel, is 
therefore, like the statement of relative calorific power, only an expression of the rela¬ 
tive capabilities, and it indicates in this respect a limit which, though it cannot be 
exceeded in any case, is never fully attained in practice. 
In order to ascertain what portion of the heat resulting from the combustion of carbon 
and hydrogen is available for producing steam, it is necessary to consider what are the 
conditions under which fuel is usually burnt, and what becomes of the heat generated 
in the two cases. In making this inquiry it is also necessary to remember that the 
several substances concerned in the combustion of fuel require different quantities ot 
heat to produce equal increments of temperature in equal weights, as stated in the fol¬ 
lowing table:— 
Quantities of Heal. 
Heat units. 
One 
pound 
of 
r Carbonic acid gas requires 
Nitrogen „ 
Atmospheric air „ 
c Steam „ 
Water „ 
, Water at 212° F. „ 
•217 h 
•245 To raise its temperature 
*238 from T to 
. -475 | T x 1 ° F. 
. roooJ 
9G6T00 for conversion into steam. 
