of Determining Moisture and Carbonic Acid in Air. 323 



made the first approximately correct analyses, reduced the 

 amount to 6'8 vols. ; and De Saussure * had, about 1830, 

 brought it down to a mean of about 4*1 vols, for country air. 

 This result was confirmed by Watson (1834) and Boussin- 

 gault (1844), who used other methods, and for long was 

 supposed to be nearly correct. Still more recent researches, 

 particularly those of Angus Smith f in Scotland, Schultze in 

 Germany, Eeiset % in France, and Muntz and Aubin in 

 France § and other countries, have gradually reduced it by 

 about a third more. 



There still, however, exists a considerable amount of ap- 

 parent contradiction on the subject. Not only are the mean 

 results of different observers different, but there is perhaps 

 still greater want of agreement as regards the limits within 

 which the proportion of C0 2 varies. That the uncertainty 

 and want of agreement which prevails is probably due to the 

 imperfection of some, at least, of the methods employed will, 

 we believe, be evident from the experiments to be recorded 

 below. The subject of the variations in the carbonic acid of 

 free air will, however, be taken up in a future paper. 



The methods at present in use for the determination of 

 carbonic acid in air are, practically speaking, of two kinds : 

 (1) those in which the C0 2 is absorbed by standardized 

 baryta- water, which is afterwards re titrated ; (2) those in 

 which it is absorbed by potash, and afterwards liberated by 

 acid and measured in the form of gas||. 



The method best known in this country is that of Petten- 

 kofer TI, or, more correctly, of Dalton** and Pettenkofer. In 

 this method a large bottle is pumped full of the air to be 

 examined. A measured quantity of standardized baryta- 



* Annales de Chimie, xliv. p. 5. 



f ' Air and Rain,' p. 59. 



J Annales de Chimie, xxvi. p. 145 (1882). 



§ Ibid. xxvi. p. 222 (1882), xxx. p. 238 (1883). 



|| A valuable critical account of the methods hitherto used is given by 

 Blochmann, Liebig's Annalen, ccxxxvii. p. 39 (1887). 



5| Abhandl. der techn. Commission der Bayer. Akad. ii. p. 3 (1858) ; 

 and Liebig's Annalen, Suppl. ii. p. 23 (1861). 



** The form finalty given to Dalton's method is described in a paper by 

 Watson, communicated by Dalton to the British Association (Brit. 

 Assoc. Report, 1834, p. 583). Watson used a large bottle, which he 

 filled with air with the help of a pair of bellows. He then added lime- 

 water, and after shaking allowed the bottle to stand for several days, at 

 the end of which time he filtered oft the carbonate, and titrated with 

 dilute sulphuric acid. Pettenkofer independently invented a similar but- 

 improved method. He introduced baryta- water as an absorbent, and 

 turmeric as an indicator, and avoided the filtration. Watson's mean 

 result for country air (4*1 vols.) was about the same as that obtained by 

 the earlier observers who used Pettenkofer's method. 



