324 Messrs. Haldane and Pembrey on an Improved Method 



water is then added, and the bottle closed and shaken, so as 

 to bring the air in contact with the baryta. After a certain 

 time the baryta solution is again titrated, and the C0 2 esti- 

 mated from the loss of alkalinity of the baryta-water. 



For certain cases where the bottle method is not suitable, 

 Pettenkofer recommends that a measured quantity of air 

 should be allowed to bubble through a long tube placed nearly 

 horizontally, and containing a known quantity of baryta-water. 

 The C0 2 absorbed by the baryta is estimated in the same way 

 as in the first method. Pettenkofer found that about 5 litres 

 per hour is the maximum rate at which the air can be passed. 



Reiset* has recently devised an apparatus which allows 

 large quantities of air to be aspirated through baryta-water. 

 With this apparatus he was able to aspirate as much as 100 

 litres through 300 cub. cent, of baryta-water in an hour. The 

 absorption of C0 2 appears to have been complete. Each 

 experiment lasted several hours. He used an enormous 

 aspirator, mounted on a cart, and capable of holding 600 litres 

 of water. 



The method in which the C0 2 is absorbed by potash, and 

 afterwards liberated by acid and measured in the form of gas, 

 was in its first form used by Mangon and Tissandierf for 

 balloon experiments. A rough form of this method has been 

 for some years in use at the Paris Observatory. The method 

 has been brought to great perfection by Muntz and Aubin|, 

 whose test-experiments show that an accuracy to about 2 per 

 cent, of the C0 2 estimated may be attained when 200 litres of 

 air are used. For absorbing the C0 2 they use tubes about a 

 metre long, filled with pumice soaked in caustic-potash solu- 

 tion free from C0 2 . After the experiment the C0 2 is liberated 

 by the addition of acid, and collected over mercury with the 

 help of a mercury-pump. The apparatus and manipulations 

 required are very complicated, so that the method is not likely 

 to come into general use. Muntz and Aubin confirmed and 

 extended Beiset's conclusions, and found that the mean pro- 

 portion of C0 2 in country air is about 2*85 vols, per 10,000, 

 and that it very seldom rises above 3*1 or falls below 2*6. 

 The experiments of these three observers are certainly the 

 most reliable of any hitherto published. 



Gravimetric methods of determining C0 2 in air were at one 

 time much employed, although they have been practically 

 disused since the introduction of Pettenkofer's method. In 



* Log. cit. p. 164. 



t Comptes Rendus, lxxx. p. 976 (1875). 



\ Annales de Chemie, xxvi. p. 222 (1882). 



