148 PARADAY 



prevent their mixing; I take the plate away, and the gases 

 are creeping one into the other. "What happens?" say 

 you; "they together produce no such combustion as was 

 seen in the case of the candle. ** But see how the presence 

 of oxygen is told by its association with this other sub- 

 stance. (") What a beautifully colored gas I have obtained 

 in this way, showing me the presence of the oxygen! In 

 the same way we can try this experiment by mixing common 

 air with this test-gas. Here is a jar containing air such 

 air as the candle would burn in, and here is a jar or bottle 

 containing the test-gas. I let them come together over 

 water, and you see the result: the contents of the test-bottle 

 are flowing into the jar of air, and you see I obtain exactly 

 the same kind of action as before, and that shows me that 

 there is oxygen in the air the very same substance that has 

 been already obtained by us from the water produced by 

 the candle. But then, beyond that, how is it that the candle 

 does not burn in air as well as in oxygen? We will come 

 to that point at once, I have here two jars; they are filled 

 to the same height with gas, and the appearance to the eye 

 is alike in both, and I really do not know at present which 

 of these jars contains oxygen and which contains air, 

 although I know they have previously been filled with these 

 gases. But here is our test-gas, and I am going to work 

 with the two jars, in order to examine whether there is any 

 difference between them in the quality of reddening this 

 gaSo I am now going to turn this test-gas into one of the 

 jars, and observe what happens: There is reddening, you 

 see; there is, then, oxygen present. We will now test the 

 other jar; but you see this is not so distinctly red as the 

 first, and, farther, this curious thing happens: if I take 

 these two gases and shake them well together with water, 

 we shall absorb the red gas ; and then, if I put in more of 

 this test-gas and shake again, we shall absorb more; and I 

 can go on as long as there be any oxygen present to pro- 

 duce that effect. If I let in air, it will not matter; but the 

 moment I introduce water, the red gas disappears; and I 



16 The gas which is thus employed as a test for the presence of oxygen is 

 the binoxide of nitrogen, or nitrous oxide. It is a colorless gas, which, when 

 brought in contact with oxygen, unites with it. forming hyponitric acid, the 

 red gas referred to. 



