VJ.J THE ATMOSPHERE. 8 



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such processes as combustion and respiration, it is clear that 

 the proportion of this gas in the atmosphere must vary 

 locally, being, for example, higher in a crowded space than 

 in the open country. The average proportion of carbonic 

 acid in the air may be estimated at from '03 to '04 per 

 cent, by volume ; thus ten thousand gallons of air will 

 contain between three and four gallons of carbonic acid. 

 Dr. Angus Smith has published, in his work on " Air and 

 Rain," a large number of analyses of air from various 

 localities, with the view of determining the variation in 

 the proportion of carbonic acid ; and, from his analyses, 

 the following examples are selected : — 



Percentage of Carbonic Acid in Air. 



On the Thames at London, mean, ...... '0343 



In the streets of London, „ -0380 



From the top of Ben Nevis '0327 



From the Queen's ward, St. Thomas's Hospital . . '0400 

 From the Haymarket Theatre, dress circle, at 



1130 P-M -0757 



From Chancery Court, 7 feet from ground . . . "1930 



From Underground Railway, mean, '1452 



From workings in mines, average of 339 samples . 7850 



Largest amount in a Cornish mine 2-5000 



These figures express percentages, but it is obvious that 

 they may be read as whole numbers per million. For 

 example, instead of saying that air from the streets of 

 London contains on an average '0380 per cent., it may 

 be said that a million gallons of the air contain 380 

 gallons of carbonic acid ; that a million gallons of air from 

 the Thames contain 343 gallons of the gas ; and so on. 



Since the atmosphere is constantly receivmg vast volumes 

 of carbonic acid from various sources, it might not un- 



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