Scientific Lectures. 203 



The Quantiy of Heat Developed by Combustion. ' 

 With. regard to the quantity of heat developed, it is fixed and defi- 

 nite for the same amount of chemical action. The quantity of heat 

 does not depend upon the manner of combustion ; it depends simply 

 upon the amount of chemical action. If we determine how much 

 carbon or hydrogen will unite with a given quantity of oxygen, we 

 shall have ascertained how much heat will be produced by their com- 

 bustion. ."We find that one pound of hydrogen requires eight pounds 

 of oxygen, and that one pound of carbon requires two and two-thirds 

 pounds of oxygen ; the heat evolved is exactly in this proportion. 

 Theory indicates that one part of hydrogen evolves on burning 2.66 

 times as much heat as an equal weight of carbon ; and experiments 

 on the heat evolved have brought out 2.61, showing the truth of the 

 theory ; the agreement being as near as could be expected in experi- 

 ments of so delicate a character. 



Intensity of Heat Developed by Combustion. 

 But while the quantity of the heat is fixed by the amount of chemi- 

 cal action, the quality entirely depends upon the mode of combustion. 

 The intensity of the heat depends on the rapidity of the chemical 

 action. If we shorten the time, we increase the intensity ; if we pro- 

 long the time, we diminish the temperature of the combustion. A 

 pound of iron buried in the earth, gradually undergoes oxydation, 

 becomes iron-rust, and in that process produces exactly as much heat 

 as when it is burned before the oxyhydrogen blow-pipe ; but in the 

 former case, the heat is produced in ten years, and in the latter in five 

 minutes. The quantity of heat is identical in both cases ; but its 

 intensity is in proportion to the rapidity of the combustion. 



Circumstances which Favok or Retard Combustion. 



There are several circumstances which favor or retard combustion. 

 One of these is temperature. Every combustible substance has what 

 we call its burning point — the temperature at which it begins to burn. 

 A few substances combine with oxygen at ordinary temperatures. 

 For instance, phosphoretted hydrogen, when allowed to escape into 

 the atmosphere, takes fire spontaneously. Zinc methyl takes fire 

 spontaneously. In other words, their burning points are below the 

 ordinary temperature of the atmosphere. 



Some bodies have two kinds of combustion, which take place at 

 different temperatures. Phosphorus, for instance, in the open air, is 

 found to shine in the dark, undergoing slow combustion, producing 

 phosphorous acid. If we allow the temperature to rise to 140° Fahr., 



