8 Composition of the Atmosphere 



In considering the heterogeneous results reported by Priestley, it is 

 important to note that the values he obtained were all comparative rather 

 than absolute. He supposed that all samples of air had different quan- 

 tities of dephlogisticated air in them; and if he took one sample of good air, 

 compared it with a sample of questioned purity taken at the same time 

 and at another place, and found that they underwent the same contrac- 

 tion in volume, he could assume that the two samples were equally pure. 

 In other words, Priestley evidently failed to realize the significance of the 

 contraction in volume. All of the observations of Priestley, then, were 

 made distinctly upon the comparative rather than upon the absolute ba- 

 sis. He was, indeed, somewhat disturbed by the fact that air which theo- 

 retically was bad did not often show any deterioration. 1 He says on this 

 point : 



When I first discovered the property of nitrous air as a test of the wholesomeness of 

 common air, I flattered myself that it might be of considerable practical use, and par- 

 ticularly that the air of distant places and countries might be brought and examined to- 

 gether with great ease and satisfaction; but I own that hitherto I have been rather 

 disappointed in my expectations from it. My own observations have not, indeed, been 

 many; but according to them the difference of the open air in different places, as indicated 

 by a mixture of nitrous air, is generally inconsiderable; and I have reason to think that 

 when very unwholesome air is conveyed to a great distance, and much time elapses before 

 it is tried, it approaches, by some means or other, to the state of wholesome air. At 

 least such I have found to be the case with the worst air that has at any time been sent 

 to me in Wiltshire from distant manufacturing towns and workshops, etc., in them, where 

 the air was thought to be peculiarly unwholesome. I am satisfied, however, from my 

 own observations, that air may be very offensive to the nostrils, probably hurtful to the 

 lungs, and perhaps also in consequence of the presence of phlogistic matter in it, without 

 the phlogiston being so far incorporated with it, as to be discoverable by the mixture 

 of nitrous air. 



I gave several of my friends the trouble to send me air from distant places, especially 

 from manufacturing towns, and the worst they could find to be actually breathed by the 

 manufacturers, such as is known to be exceedingly offensive to those who visit them; but 

 when I examined those specimens of air in Wiltshire, the difference between them and 

 the very best air in this country, which is esteemed to be very good, as also the difference 

 between them and specimens of the best air in the counties in which those manufacturing 

 towns are situated, was very trifling. 



Mr. S. Vaughan, senior, on his passage from Jamaica, brought me two bottles of air, 

 one from the hold of the ship, intolerably offensive, the other the fresh air above deck 

 in about 30' N.; but the difference between these specimens of air, and the air of Wilt- 

 shire, was quite inconsiderable. 



I have frequently taken the open air in the most exposed places in this country at 

 different times of the year, and in different states of the weather, etc., but never found 

 the difference so great as the inaccuracy arising from the method of making the trial 

 might easily amount to, or exceed. 



This recognition of at least the existence of a limit of accuracy for his 

 method was unfortunately not seriously considered by many of his con- 

 temporaries. 



1 Priestley, loc. cit., 1779, 4, p. 269. 



