Ozone and Ozone Tests. 161 



Locally it varies considerably, being most abundant near the 

 sea and in high, mountainous districts, and least so in cities and 

 large towns. There is more ozone at Silloth, a seaport town 

 in Cumberland, than in any other British station ; whilst in 

 Scotland the amount is very great at Braemar, a mountainous 

 town near the Queen's residence at Balmoral. On the other 

 hand, at Manchester and in London very little exists. Then, 

 again, there are periods with much ozone, and periods with 

 scarcely any. An instance of this occurs in the month of 

 January just passed; up to the 19th scarcely any ozone was 

 present, whilst from the 19th to the end of the month the 

 amount was considerable. 



No doubt, circumstances act for or against the development 

 of ozone, at one time augmenting the amount, and at another 

 diminishing it. Chemical action increases with an increase of 

 heat, and diminishes with an increase of cold ; to this cause is 

 probably owing the absence, more or less, of ozone in frosty 

 weather. Moisture to a certain extent is favourable to chemical 

 action, yet an excess of moisture acts in the opposite direction ; 

 there is less ozone with a very dry air, and still less with one 

 completely saturated with moisture. It must be borne in mind 

 that we frequently have the air saturated with water, whilst the 

 atmosphere never approaches perfect dryness ; on the driest 

 days a considerable amount of water is present in the air. 

 There is a striking difference in different directions of the 

 wind, for there is least ozone with a N.E. wind, and most 

 with one between S.W. and S.S.W. ; the latter contains air 

 much charged with moisture, whilst the former is more or less 

 dry ; then, again, as a rule, the S.W. wind is brisk, whilst the 

 N.E. wind is sluggish. An increase in the pressure of the wind, 

 which is synonymous to an increase in the velocity of the air, 

 is attended with an increase in ozone, as registered on the test- 

 slip ; yet it does not follow that there is an actual increase, 

 because if the same amount is present to-day as yesterday, and 

 if to-day the velocity of the air is five times greater than yes- 

 terday, it will be apparent that five times the amount of air 

 charged with ozone must pass over the test-slip, and this will, 

 no doubt, increase the colour of the test. 



It seems somewhat singular, that the lower the barometer 

 falls the more does ozone develop itself; that at a pressure of 

 29 inches there is considerably more ozone than with one at 30 

 inches. Let us consider what this difference means : when the 

 barometer is at 30 inches, the air is capable of balancing a 

 column of mercury 30 inches in length, whilst when it is only 

 29 it can only balance one of 29 inches ; with the barometer at 

 29 inches, a cubic foot of air at a temperature of 10° weighs 

 about 573 grains; at 30° about 549 grains; at 50° about 526 



