Scientific Lectures. 201 



The Reciprocal Nature of Combustion. 



I have stated that we call certain bodies combustible, and others, 

 supporters of combustion ; but this is simply a relation -which is 

 reciprocal. As our atmosphere contains oxygen, we call bodies which 

 combine with oxygen combustible, and oxygen a supporter of com- 

 bustion ; but were the atmosphere composed of hydrogen, instead of 

 oxygen, we should then regard oxygen as combustible, and hydrogen 

 as the supporter of combustion. I have here an apparatus by which 

 I propose to show you that oxygen is combustible, and hydrogen the 

 supporter of combustion. This large jar is filled with hydrogen gas, 

 and by introducing a jet of oxygen within it, and heating it to the 

 proper point, you see that there is a flame of oxygen gas burning in 

 the hydrogen, reversing the usual order of combustion. "Water is the 

 result of the combination, and the sides of the jar are already dimmed 

 with the moisture produced by the combustion of the oxygen. 



A few moments ago, I showed you chlorine as a supporter of the 

 combustion of phosphorus, sodium and antimony. I now introduce 

 a jet of chlorine into the hydrogen gas, and you see the combustion 

 of the chlorine, producing hydrochloric acid. 



I have here a jar filled with street gas, which, you will see, may 

 also be made to support combustion. I first introduce the jet of 

 oxygen gas, and you observe the flame of the oxygen, and the gradual 

 condensation of the moisture produced, upon the jar. Even the char- 

 acter of the flame is the same as that of a candle ; for, in the interior, 

 we have the blue cone from the excess of oxygen, and, on the exterior, 

 the yellow cone from the excess of carbon. 



I now introduce a jet of chlorine, which is also consumed in the 

 street gas, with the separation, however, of carbon, which refuses to 

 unite with the chlorine. 



To still further illustrate the reciprocal character of the phenomena 

 of combustion, I have here a vessel containing boiling aqua ammonia, 

 which evolves ammoniacal gas, a compound of hydrogen and nitro- 

 gen. Introducing the jet of oxygen, you see that we have a flame 

 of oxygen burning in an atmosphere of ammonia ; and now, allow- 

 ing this same ammonia to escape by this jet above, and lighting it, 

 we have also the flame of the ammonia burning in an atmosphere 

 containing oxygen, the two flames illustrating the reciprocal nature 

 of combustion. 



The Products of Combustion. 

 Oxygen is the predominant element in nature. Nearly everything 

 we see, excepting animals and plants, is already burned. The 



