528 Dr. Andrews's Report on the Heat qf Combination. 



bonic oxide, the resulting compound is obtained in the gaseous state, 

 Avhile in the combustion of hydrogen it is condensed during the » 

 course of the experiment into a liquid; and if, from the entire quan-'» 

 tity 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. Pro- 

 toxide 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. 



Sulpliur, copper and the protoxide of copper, disengage, during 

 their combustion 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 tlie bodies already enumerated ; and potassium a 

 still larger quantity than zinc. The combustion of a large number 

 of carbo-hydrogens, alcohols, aethers and organic acids has been ex- 

 amined by Fabre and Silbermann. Their results prove the opinion 

 to be erroneous, that if we subtract the oxj'gen 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 

 evolution 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 pliaenomena 

 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 influenced to a considerable extent by the physical state in which 

 the combustible exists before combination. According to their ex- 

 periments, carbon in the form of the diamond disengages 7824 units 

 of heat during its combustion in oxygen gas ; in the form of gra- 

 phite 7778 units; and in that of wood-charcoal 8080 units. Ac- 

 cording to my own experiments and those of Despretz, the combus- 

 tion of wood-charcoal produces only about 7900 units. Fabre and 

 Silbermann have also supposed that they were able to detect differ- 

 ences in the quantities of heat disengaged by sulphur in its differ- 

 ent allotropic states. The same chemists have also made the re- 

 markable observation, that a much larger quantity of heat is evolved 

 by the combustion of carbon in the protoxide of nitrogen than in 

 oxygen gas. PVoni 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 ap- 

 paratus, 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 ; 



