190 ANNUAL OF SCIENTIFIC DISCOVERY. 



ordinary distinctions into fractious, and a fraction always appears 

 small. It would have been better to have stated the amount of 

 gases in all cases in a million parts. If it required nearly a per 

 cent, of impurity to do us any injury it might be convenient to 

 use per cents., but, so far as I can make out, one of the greatest 

 causes of difference in the air of our large cities exists in an 

 amount which cannot be stated in whole numbers unless a 

 million be taken as the amount for comparison. If we even take 

 the air of the underground railway tunnel, we find that the sul- 

 phurous acid amounts to only 3 times the amount, when the sul- 

 phur taste is pretty strong. This gives an indication of the limit 

 of the senses in relation to this gas. It must be between the 

 amount which exists in London and that which exists in the 

 tunnel. 



" Even taking oxygen alone, the amount of variation gives much 

 higher numbers than we get with sulphur acids, but there is still 

 an appearance of trifling when we speak of 20.99 and 20.79 or 

 such numbers ; still they do indicate atmospheres such as all sound 

 sense of smell must distinguish. If we say 209,900 in a million 

 and 207,900 in a million, we see the distinction clearly made with 

 2,000 units for the intermediate stages." 



Carbonic acid. "The variations in the amount of carbonic 

 acid are less than in the case of oxygen. If we have the highest 

 average amount of oxygen obtained on land 209,990, and the 

 lowest not known to be effected by a town, 209,470, we have a 

 difference of 520 in a million ; but if we treat similarly the car- 

 bonic acid, taking it where fields begin near Manchester to the top 

 of Ben Nevis, or to the London parks, we have a similarity which 

 is very remarkable. All are about 333 in a million. This has 

 been referred to the fact that the method of oxygen analysis is less 

 easy ; but the results are so consistent that we may give up that 

 idea. It is rather different when we go to close places ; the 

 amount of carbonic acid shows itself there more decidedly, per- 

 haps because it does not diffuse so rapidly as the oxygen. The 

 oxygen is, on the whole, not so practical an index to the condition 

 of the air as the carbonic acid is." Some of the amounts of 

 oxygen and of carbonic acid obtained follow : 



Oxygen. Parts in a million 



N. E. sea-shore and open heath, Scotland, 209,990 



Top of hills, Scotland, 209,800 



Suburb of Manchester in wet weather, 209,800 



Manchester in fog and frost, 209,100 



London, open places in summer, 209,500 



London, average of 68 determinations, 208,850 



Pit of theatre, 11.30 P.M., 207,400 



Metropolitan railway, - 207,000 



Court of Queen's Bench, Feb. 2, 18G6, 200,500 



In mines, 204,240 to 201,400 



Worst specimen yet examined in a mine, 182,700 



Carbonic acid. 



In mines largest amount found in Cornwall, 25,000 



" average of 339 analyses 7,8.">0 



Manchester during fogs, C79 



