of Determining Moisture and Carbonic Acid in Air. 319 



by Germany and Russia, are all reduced from Regnault's 

 table of pressure and psychroinetric formula. 



In conclusion may be given the results obtained by other 

 observers, who have compared the psychrometer with the 

 chemical method. Regnault *, who made 106 experiments 

 under very varied conditions, obtained for the mean per- 

 centage error 2, whilst the extreme percentage error varied 

 from + 12 to — 10. In order to calculate out the results 

 from the readings of the dry and wet bulbs, Regnault 

 determined the values for the constant A in each series of 

 experiments. Each experiment lasted about one hour ; the 

 readings of the psychrometer were taken every five minutes, 

 and from the mean of these the result was calculated. M. 

 Izarn made 34 comparative experiments in the Pyrenees. 

 These are given by Regnault in his paper -j-. The extreme 

 percentage differences are + 2 and — 3 in the first series, 

 and +- 10 and — 10 in the second. 



Shaw made comparisons in a room with a current of air 

 passing over the instruments ; he does not, however, consider 

 them satisfactory. The tensions calculated from the psychro- 

 meter by Glaisher's Tables were generally higher than those 

 given by the chemical determination, the variations ranging 

 from + 30 per cent, to — 7 per cent. }. 



III. The Determination of Carbonic Acid. 



The method adopted by us for carbonic acid is similar in 

 principle to that for moisture. The absorption-apparatus 

 (see fig. 1) consists of a pair of test-tubes arranged in the 

 same way as was described above (p. 307) §. The first tube is 

 filled with soda-lime, and the second with sulphuric acid and 

 pumice. The air must arrive perfectly dry at the absorption- 

 tube for carbonic acid, and is therefore passed through the 

 previously described absorption-apparatus for moisture. The 

 connecting piece of rubber tubing should be as short as 

 possible. 



The soda-lime we have hitherto used was made by heating- 

 together in a copper vessel 1200 grans, of caustic soda in 

 strong solution, and the product obtained by slaking to a fine 

 powder 1000 grins, of quicklime. The heating was continued 

 until the soda-lime became capable of being broken up into 

 fragments in a mortar. The fragments were rapidly sifted 



* Annates de Chimie, xxxvii. pp. 2G4-285 (1835). 

 t Ibid. pp. 275-272. 

 % Phil. Trans. 1888, A, p. 111. 



§ The same counterpoise is used for the carbonic acid as tor the 

 moisture determinations. 



