102 LOUIS PASTEUR 



through the vessel could be traced by the illumi- 

 nated motes in the air. After the platinum coil 

 had been heated to redness for some time the 

 course of the beam was no longer visible. The 

 floating matter had been burned; it was therefore 

 organic matter. The same result was also obtained 

 by passing air through the flame of an alcohol 

 lamp. Examination of floating matter in several 

 places has shown that it contains as a rule only a 

 small proportion of inorganic dust. The more 

 persistent floating matter is composed mostly of 

 organic material which when dried and finely 

 divided is very light and readily wafted to great 

 heights by even a very gentle breeze. 



Occasionally inorganic particles, if very small 

 and of light material, may be carried to great dis- 

 tances in the air. Several years ago after the 

 eruption of the great volcano of Krakatao, it was 

 estimated that the dust was carried several times 

 around the world in the higher levels of the atmos- 

 phere, causing the striking red sunsets which were 

 observed for several weeks after that event. 



Tyndall found that passing air through sulphuric 

 acid as Schulze had done failed to make it "op- 

 tically pure." Some of the motes in the bubbles 

 failed to come into contact with the liquid and 



