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ciated by Laplace, that those quantities of a combustible 
body which require an equal amount of oxygen for combus- 
tion, evolve also an equal quantity of heat. The oxygen 
required in the combustion of a fuel thus becomes a 
standard by which its relative heating value can be de- 
duced. 
That greatest of modern chemists, Berthier, has founded 
upon this law a practical process as follows. A weighed 
quantity of the fuel very finely pulverised is intimately mixed 
with about 1,500 grains of litharge (oxide of lead, Pbo), and 
the whole introduced into an earthen crucible, the cover 
of which is luted down with fire clay, and, when dried, 
allowed to remain about fifteen minutes in an assay furnace, 
with a moderately strong fire. The combustible will reduce 
a certain quantity of the oxide of lead to metallic lead by 
uniting with its oxygen, in the same manner in which a 
body burns in atmospheric air by the abstraction of its 
oxygen. The number of equivalents of oxygen required to 
effect such combustion will, therefore, be accurately indi- 
cated by the number of equivalents of reduced metallic 
lead, which can be easily estimated from the weight of 
the button produced. 
According to the experiments of Despretz, 1 equivalent 
of carbon is sufficient to heat 78.15 equivalents of water 
from 0° to 100° Centigrade; and as the same quantity of 
carbon requires 2.666 equivalents of oxygen, which taken 
from litharge leaves 34.5 equivalents of metallic lead, we 
have for the result, that every equivalent of lead that is 
reduced by an experiment with any kind of fuel, corres- 
ponds to ^| = 2.265 equivalents of water, which will be 
raised from 0° to 100°. But this method is merely approxi- 
mate, and obviously, from the amount of hydrogen in the 
fuel above what is required to form water with the oxygen 
not being taken into consideration, very serious error may be 
