162 Ozone and Ozone Tests. 



grains ; at 70' about 504 grains, and at 90° about 482 grains ; 

 with the barometer at 30 inches, below the freezing point, it 

 will be about 20 grains heavier ; at a temperature of 50°, 18 

 grains heavier ; and at 90°, 16^ grains heavier. Supposing the 

 atmosphere to be completely saturated with moisture, the 

 weight of vapour in a cubic foot of air, at a temperature of 30°, 

 will be 2 grains ; at 50", 4 grains ; at 70^, 8 grains ; and at 90°, 

 nearly 15 grains. The amount of vapour differs, according to 

 the difference between the wet and dry bulb thermometer; taking 

 the greatest difference, or driest point at each temperature, the 

 weight of vapour at 30° will be 1 grain ; at 50°, 2 grains ; at 

 70°, 4 grains ; and at 90°, 6 grains ; so that at least six times as 

 much moisture exists in the hottest days as in the coldest ; we, 

 however, feel the air to be drier, because at a temperature of 

 30° only 1 grain extra is required for perfect saturation, whilst 

 at 90° it would require 8 or 10 grains extra. 



If the air were completely saturated with moisture, the 

 whole amount of water in a vertical column of the atmosphere 

 would be 2j inches at a temperature of 30°, and 19| inches at 

 a temperature of 90° ; but if the air were unusually dry, the 

 amount would be \\ inches at 30°, and 10 inches at 90°. This 

 little analysis of the atmosphere will give an insight into the 

 subject ; yet it does not afford us a clue to the reason why more 

 ozone exists with the barometer low, unless we consider that, 

 as wind accompanies a low state of the barometer, it is to be 

 attributed to this cause alone. 



From what has now been said, it becomes evideut that 

 certain corrections will become necessary before the actual 

 amount of ozone can be determined. 



We next come to the test used in these investigations, which » 

 was the subject of much discussion at the Cambridge meeting 

 of the British Association. Great difficulty has always been 

 experienced in producing tests that should all register alike ; if 

 we expose those of Schonbein and Moffatt's together, we do not 

 get the same result, and even tests made by tho same persons 

 at two different times will also not read alike, and my investi- 

 gations wen; with the view of finding out tho cause of this. 



Commencing at tho very beginning of their manufacture, it at 

 once occurred bo me thai tlie starch of commerce of which they 

 wero made could not be sufficiently pure for such delicate ex- 

 periments; in their manufacture, lime, sulphuric acid, and 

 chlorine are used— substances which will themselves colour the 

 tests without I lie aid of ozone. Place under a bell-glass a small 

 cup of chloride of lime, and under another a piece of limestone, 

 on which sulphuric acid has been poured, and under these glasses 

 suspend an ozone test; a few minutes will suffico to demon- 

 strate this. The ordinary iodide of potassium is impure ; and 



