JOSEPH PRIESTLEY 97 



did not, in the least, diminish common air, which I have observed that 

 nitrous air, in this state, in some measure does. 



But I was much more surprised, when, after two days, in which 

 this air had continued in contact with water (by which it was dimin- 

 ished about one-twentieth of its bulk) I agitated it violently in water 

 about five minutes, and found that a candle still burned in it as well as 

 in common air. The same degree of agitation would have made 

 phlogisticated nitrous air fit for respiration indeed, but it would cer- 

 tainly have extinguished a candle. 



These facts fully convinced me, that there must be a very material 

 difference between the constitution of air from mercurius calcinatus, 

 and that of phlogisticated nitrous air, notwithstanding their resem- 

 blance in some particulars. But though I did not doubt that the air 

 from mercurius calcinatus was fit for respiration, after being agitated 

 in water, as every kind of air without exception, on which I have tried 

 the experiment, had been, I still did not suspect that it was respirable 

 in the first instance ; so far was I from having any idea of this air 

 being, what it really was, much superior, in this respect, to the air of 

 the atmosphere. 



In this ignorance of the real nature of this kind of air, I continued 

 from this time (November) to the ist of March following; having, in 

 the meantime, been intent upon my experiments on the vitriolic acid 

 air above recited, and the various modifications of air produced by 

 spirit of nitre, an account of which will follow. But in the course of 

 this month, I not only ascertained the nature of this kind of air, 

 though very gradually, but was led to it by the complete discovery of 

 the constitution of the air we breathe. 



Till this 1st of March, 1775, I had so little suspicion of the air from 

 mercurius calcinatus, &c., being wholesome, that I had not even 

 thought of applying it to the test of nitrous air; but thinking (as my 

 reader must imagine I frequently must have done) on the candle 

 burning in it after long agitation in water, it occurred to me at last to 

 make the experiment ; and putting one measure of nitrous air to two 

 measures of this air, I found, not only that it was diminished, but 

 that it was diminished quite as much as common air, and that the red- 

 ness of the mixture was likewise equal to that of a similar mixture of 

 nitrous and common air. 



After this I had no doubt but that the air from mercurius calcinatus 



