
ON THE HEAT OF COMBINATION. a7 
But if we apply corrections for the heat due to the changes of physical state 
which occur in some of these reactions, the same agreement will no longer 
be observed. Thus in the combustion of carbonic oxide, the resulting com- 
pound is obtained in the gaseous state, while in the combustion of hydrogen 
it is condensed during the course of the experiment into a liquid; and if, 
from the entire quantity of heat evolved in the latter case, we deduct that 
arising from the condensation of the vapour of water, the result will no 
longer agree with the quantity of heat obtained in the former case. Protoxide 
of tin may probably be added to the foregoing list,and perhaps also phosphorus, 
which disengages however a little more heat than the other bodies. 
Sulphur, copper and the protoxide of copper, disengage, during their com- 
bustion in oxygen gas, a little more than half the quantity of heat evolved 
by the preceding class of bodies. Carbon occupies an intermediate position, 
while zinc gives out a larger quantity of heat than any of the bodies already 
enumerated ; and potassium a still larger quantity than zinc. The combus- 
tion of a large number of carbo-hydrogens, alcohols, zethers and organic acids 
has been examined by Fabre and Silbermann. Their results prove the opi- 
nion to be erroneous, that if we subtract the oxygen in the form of water, 
the remaining elements give the same amount of heat as in the free state. 
In the reduction of oxide of iron by hydrogen gas, no perceptible evolu- 
tion of heat occurs, while in the reduction of the oxide of copper by the 
same gas, it is well known that ignition takes place, unless the experiment is 
conducted very slowly. These phenomena are at once explained by the fact, 
that in combining with oxygen, hydrogen gas disengages nearly the same 
quantity of heat as iron, and twice as much heat as copper. 
Fabre and Silbermann have observed that the heat of combustion is influ- 
‘enced to a considerable extent by the physical state in which the combustible 
exists before combination. According to their experiments, carbon in the 
form of the diamond disengages 7824 units of heat during its combustion in 
oxygen gas; in the form of graphite 7778 units; and in that of wood-char- 
coal 8080 units. According to my own experiments and those of Despretz, 
the combustion of wood-charcoal produces only about 7900 units. Fabre 
and Silbermann have also supposed that they’ were able to detect differences 
in the quantities of heat disengaged by sulphur in its different allotropic 
states. The same chemists have also made the remarkable observation, that 
a much larger quantity of heat is evolved by the combustion of carbon in the 
protoxide of nitrogen than in oxygen gas. From this it should follow that 
in the separation of the elements of the protoxide of nitrogen, heat would be 
set free. Accordingly, by passing the protoxide of nitrogen through a pla- 
tina tube heated to redness by burning charcoal in a suitable apparatus, it 
was found that a larger quantity of heat was actually evolved than could be 
accounted for by the weight of charcoal burned. 
_ Combustions in Chlorine Gas.—Some years ago, I published the results of 
‘an investigation on the quantities of heat evolved in the combination of zinc 
and iron with chlorine, bromine and iodine; and I have lately given an ac- 
count of a set of experiments on the combustion of potassium, tin, antimony, 
mercury, phosphorus and copper in chlorine gas. So far as I am aware, the 
‘only other experiments on this subject are those described by M. Abria on 
‘the combustion of hydrogen and phosphorus in chlorine. From a com- 
parison of the results, it appears that in several cases the quantities of heat 
‘evolved during the combustion of the same metal in oxygen and chlorine are 
nearly the same. This observation applies particularly to the cases of iron, 
tin and antimony. Zinc however disengages a greater quantity of heat with 
chlorine (6309 units) than with oxygen (5366 units), and copper nearly twice 
