COMMON THINGS FIRE. 



When the combustion has ceased and the gas contained in the 

 flask has cooled, it will be found that the mercury in the neck of 

 the flask 'will stand at exactly the same elevation as it did before 

 the combustion. The gas contained in the flask has therefore the 

 same volume as before, nevertheless it is easy to show that it is by 

 no means the same gas. 



In the first place, if it be weighed, it will be found to weigh 22 

 instead of 16 grains ; and if the unburned residue of the carbon be 

 weighed, its weight will be found to be 6 grains less than it was 

 before the experiment. The inference is, that 6 grains of the 

 carbon have combined with the 16 grains of the oxygen previously 

 contained in the flask, but that in thus combining, the carbon has 

 not made any change in the volume of the gas. 



If the gas contained in the flask be examined by the usual tests, 

 it will immediately appear that it is no longer oxygen. No com- 

 bustible will burn in it, and it will not support life by respiration. 

 In fine, it will be found to be identical with the noxious gas called 

 choke-damp, and to possess all the chemical characters of the gas 

 called CARBONIC ACID. 



If the same flask, similarly filled with nitrogen gas or azote,* 

 be submitted to a like experiment, the result will not be the same. 

 The solar rays concentrated on the charcoal will still render it 

 red hot, but it will not burn nor undergo any other change. On 

 removing the focus of solar rays from it, it will become gradu- 

 ally cool, and when removed from the flask will have the same 

 weight as when introduced into it. The azote which fills the 

 flask will also be found to be unaltered. 



It follows, therefore, that the FIHE produced when carbon 

 burns in common air is nothing more than the heat and light 

 developed in the formation of carbonic acid, by the combination 

 of the carbon with the oxygen of the surrounding air, and that 

 these substances combine in the proportion of 6 parts by weight 

 of carbon to 16 of oxygen.f 



9. It has been already shown}; that hydrogen combines with 

 oxygen in the proportion of 1 part by weight of the former to 8 of 

 the latter to form water, and that if the combination be formed in 

 a pure or nearly pure atmosphere of the gases it is instantaneous 

 and accompanied by an explosion. If, however, the combination 

 take place, as it may, in common air, the phenomena will be very 

 different. 



If pure hydrogen, compressed in a bladder or other reservoir, 

 be allowed to issue from a small aperture, a light applied to it 



* See Tract on Air. 

 f More precisely 6*04 or 6*12 of carbon to 16 of oxygen. 



J See Tract on Water. 

 196 



