64 PUBLIC HEALTH CHEMISTRY 



METHODS. A large jar is filled with the air sample as 

 directed above. Fifty c.c. of the baryta water are added, 

 the jar is capped, and shaken now and then over a period 

 of half an hour. Thereafter 25 c.c. are removed from the 

 jar and titrated with the standard oxalic, using phenol- 

 phthalein as indicator until the red colour is just discharged. 

 Twenty-five c.c. of fresh baryta are similarly titrated, and 

 the number of c.c. required noted. The difference between 

 this latter and the number of c.c. required for the 25 c.c. 

 removed from the jar, measures in oxalic the amount of 

 baryta which has had its alkalinity neutralized by absorp- 

 tion of CO 2 from the air in the jar. This amount doubled 

 measures the quantity so neutralized in the 50 c.c. taken, 

 and as the oxalic per c.c. = 0-5 c.c. CO 2 , on multiplying 

 the result by 0-5 we get the number of c.c. of CO 2 absorbed 

 from the volume of air in the jar. The CO 2 is at normal 

 temperature and pressure, and the air in the jar is at the 

 temperature and pressure noted on collection of the sample, 

 and so these two volumes are not quite comparable. The 

 volume of sample in the jar must first be corrected to the 

 volume it would occupy at standard temperature and 

 pressure. This may be done after several fashions, but 

 the one here recommended is to use the formula 



V X P V X P' 



where V P and T denote respectively volume at 

 standard pressure (760 mm. or 29-92 inches of mercury) 

 and at standard absolute temperature (o C -f 273 or 

 32 F. + 459), and V, P', and T', the volume of the jar as 

 measured (less 50 c.c. displaced by the baryta water 

 added), and the pressure and temperature at the time and 

 place of taking the sample, the temperature being changed 

 to the absolute scale. 



Thereafter the proportion of CO 2 present is calculated 

 and the result expressed, which may be stated as a per- 

 centage, or parts per 1000 or per 10,000. The air of the 

 open country averages almost exactly 3 parts per 10,000. 



Haldane's Method requires a special apparatus, but once 

 facility of manipulation of the apparatus has been acquired, 

 an accurate result can be obtained in ten minutes and 



