OZONE IN THE UPPER ATMOSPHERE 469 



the measurements made with the balloons above Manchester, the 

 mean volume of ozone between ground level and altitudes up to 

 20 kilometres gave a value of 2 f i x io~°. Even after allowing 

 for the absence of this gas at lower altitudes, the measurements, 

 though only approximate, indicate that there is no very large 

 increase in the amount of ozone at altitudes between 4 and 20 

 kilometres. However, since at this last height the pressure of 

 the atmosphere is still about 4 cm., the amount of light of wave- 

 length below 200 fifi, which is necessary to form ozone, would 

 be very small. The probability thus still remains that above 

 this elevation a largely increased content of ozone prevails. 



Part 5. The Influence of Ozone on the Nature of Light 



from the Sky 



The results in the above experiments of the approximate de- 

 terminations of the quantity of ozone in the higher atmosphere 

 supply data which enabled measurements to be made in the 

 laboratory of the depth of colour given by this amount of ozone. 



For this experiment, a glass tube of 2'8 metres length and 

 4 cm. diameter was taken. The walls were provided with side 

 tubes, one near each end, to enable the passage of the ozonised 

 gas through the tube. The two ends of the main tube were 

 covered by thin plates of glass, which were cemented by sodium 

 silicate solution so as to make an air-tight connection. The out- 

 side of the tube was wrapped with black paper, and a white 

 paper disc placed over one of the end plates. On illuminating 

 this by daylight and viewing the transmitted light through the 

 other end, the intensity of coloration produced on admitting 

 ozone of known concentration could be observed. 



The ozone for this purpose was prepared from oxygen by 

 passing through a number of annular glass tubes where it was 

 exposed to the silent electric discharge produced by an induction 

 coil. After time had been allowed for the composition of the gas 

 in the sighting tube to become uniform, an analysis of the gas 

 was made by passing a measured volume into an acidified 

 potassium iodide solution. 



The results given in the table below record the observations 

 made with the tube when filled with oxygen containing different 

 concentrations of ozone. The thickness of the layer of pure gas 

 which is equivalent to this concentration is also given. 



