NITROGEN AND All; 235 



chloride for absorbing the moisture 28 is placed before the apparatus for 

 the absorption of the carbonic anhydride, and a measured mass of air 

 is caused to pass through the whole apparatus by means of an aspirator. 

 In this manner the determination of the moisture is combined with the 

 absorption of the carbonic acid. The arrangement shown in fig. 3& 

 is such a combination. 



The amount of carbonic anhydride 29 in free air is incomparably 

 more constant than the amount of moisture. The average amount of 

 carbonic anhydride in 100 volumes of dry air is approximately , ;J () - p.c. 

 that is, 10000 volumes of air contain about 3 volumes of carbonic anhy- 

 dride, most frequently about 2 -95 volumes. As the specific gravity of 

 carbonic anhydride, referred to air =1'52, therefore 100 parts by weight 

 of air contain 0'045 part by weight of carbonic anhydride. This quantity 

 varies according to the time of year (more in winter), the altitude 

 above the level of the sea (less at high altitudes), on the proximity to- 

 forests and fields (less) or cities (greater), &c. But the variation is 

 small and rarely exceeds limits of 2^ to 4 ten-thousandths by volume. 30 

 As there are many natural local influences which either increase the 



^ It is evident that the calcium chloride employed for absorbing the water should be- 

 free from lime or other alkalis in order that it should not retain carbonic anhydride. Such 

 calcium chloride may be prepared in the following manner. An entirely neutral solution 

 of calcium chloride is prepared from lime and hydrochloric acid ; it is then evaporated first 

 over a water-bath and then over a sand-bath with great care. When the solution attains, 

 a certain strength a scum is formed, which solidifies on reaching the top. This scum is 

 collected, and will be found to be free from caustic alkalis. It is necessary in any case to 

 test it before use, as otherwise a large error may be introduced into the results, owing to 

 the presence of free alkali (lime). It is better still to pass carbonic anhydride through the 

 tube containing the calcium chloride for some time before the experiment, in order to 

 saturate any free alkali that may remain from the decomposition of a portion of the 

 calcium chloride by water, CaCl 2 + 2H 2 O = CaOH 2 O + 2HCl. 



29 Recourse is had to special methods when the determination regards only the carbonic 

 anhydride of the air. For instance, it is absorbed by an alkali which does not contain 

 carbonates (by a solution of baryta or caustic soda mixed with baryta), and then the 

 carbonic anhydride is expelled by an excess of an acid, and its amount determined by the 

 volume given off. During the last ten years the question as to the amount of carbonic 

 anhydride present in the air has been submitted to many voluminous and exact researches^ 

 especially those of Reiset, Schloesing, Miintz, and Aubin, who showed that the amount of 

 carbonic anhydride is not subject to such variations as was at first supposed on the basia 

 of incomplete and insufficiently accurate determinations. 



30 It is a different case in enclosed spaces in dwellings, cellars, wells, caves, and mines,, 

 where the renewal of air is hampered. Under these circumstances large quantities of 

 carbonic anhydride may accumulate. Even in cities, where there are many conditions for 

 the evolution of carbonic anhydride (respiration, decomposition, combustion), its amount 

 is greater than in free air, yet even in still weather the difference does not often exceed one 

 ten-thousandth (that is, rarely attains 4 instead of 2'4 vols. in 10000 vols. of air). Hundreds 

 of very careful comparative determinations made simultaneously around Paris and in 

 tlu- city itself, and constant daily determinations conducted at certain meteorological 

 stations (for instance, at Montsouris, near Paris), confirm this conclusion. On high 

 mountains and in deep valleys, as has been proved in the Pyrenees, the difference of 



