360 HISTORY OF SCIENCE. 



candle to burn in it, yet, after more than ten. times as much agitation 

 as would be sufficient to produce this alteration in the nitrous air, no 

 sensible change was produced in this. A candle still burned in it with 

 a strong flame, and it did not in the least diminish common air, which 

 I had observed that nitrous air in this state (i.e., nitrous oxide] 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 di- 

 minished 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 it did in common air. The same degree of agitation would 

 have made phlogisticated nitrous air fit for respiration, indeed, but it 

 would have extinguished a candle. 



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

 difference between the constitution of the air from mercurius calrinatus 

 and that of dephlogisticated nitrous air (nitrous oxide], notwithstanding 

 their resemblance in some particulars. But though I did not doubt 

 that the air from mercurius caltinatus was fit for respiration after being 

 agitated with water, as every kind of air had been, without exception, 

 on which I had tried the experiment, 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, which 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 (Nov. 1774) to the March following (1775), having in 

 the meantime been intent upon my experiments on the vitriolic acid 

 air, and the various modifications of air produced by spirit of nitre 

 (nitric acid}. But in the course of this month I not only ascertained 

 the nature of this kind of air, though very gradually, but was led by 

 it to the complete discovery, as I then thought, of the constitution of 

 the air we breathe. 



"Till the first of March, 1775, I had so little suspicion of the air 

 from mercurius calrinatus , etc., being wholesome, that I had not even 

 thought of applying to it the test of nitrous air (nitric oxide} ; but think- 

 ing, as my reader must imagine I frequently must have done, on the 

 candle burning in it after long agitation in water" (he had found that 

 nitrous oxide is readily absorbed by water], " it occurred to me at last 

 to make the experiment, and putting one measure of nitrous air (nitric 

 oxide] to two measures of this air (/>., the oxygen), I found not only 

 that it was diminished, but that it diminished quite as much as common 

 air, and that the redness of the mixture was likewise equal tp that of 

 a similar mixture of nitrous and common air. 



" After this I had no doubt but that the air from mercurius calri- 

 natus was fit for respiration, and that it had all the properties ofgenuine 

 common air, But I did not take notice of what I might have observed, 

 if I had not been so fully possessed by the notion of there being no 



