FORMATION OF SMALL CLEAR SPACES IN" DUSTY AIR. 257 



sphere can be so full of dust, as this experiment shows it to be, as it escapes our 

 observation under ordinary conditions of lighting, and it gives us a feeling of 

 discomfort to realise that we are breathing that dust-laden air. This uneasiness 

 was by no means decreased when my experiments on cloudy condensation 

 revealed the fact that, in addition to that mass of visible dust, there are enormous 

 multitudes of particles so small that even the concentrated light of the sun does 

 not reveal them. These minute particles are so numerous that hundreds of them 

 are crowded into every cubic centimetre of air. On realising these facts our 

 feelings are those of wonder that our lungs can keep so clean as they do, while 

 such vast quantities of impurities are constantly ebbing and flowing through 

 them. At that time I was not aware that there is an influence ever at work tend- 

 ing to protect the lungs by preventing, to a certain extent, the particles of dust 

 coming into contact with their surfaces, — that nature had provided a subtle form 

 of mechanism possessing some of the advantages of a filter without any of its 

 disadvantages. The experiments here described show that a hot surface 

 repels the dust particles in the air. The heat of our bodies will, therefore, 

 exert a protective influence on the lungs, and tend to keep them free from 

 dust. 



Our lungs, however, are not only hot, they are also wet. What influence 

 will the constant evaporation which takes place at the surface of the tubes 

 and passages have on the dust? To answer this question, I fitted the flat test 

 surface in the dust-box, and through an opening in the top introduced a brush 

 dipped in water, with which one-half of the surface was kept wet, the other half 

 being dry, to compare the effects under the two conditions. When the surface 

 was heated a few degrees, to even less than the temperature of our bodies, the 

 result was most decided, the dust being driven more than twice as far from the 

 plate in front of the wet part as it was from the dry. The evaporation, there- 

 fore, of the water from the surface of the bronchial tubes tends strongly to 

 ward off the dust, and keep it from coining into contact with their surfaces. 

 We must not, however, imagine that the heat, or the heat and the evaporation, 

 are sufficient entirely to prevent the dust coming into contact with the surfaces 

 of our bronchial tubes and passages, because dust really does come into contact 

 with them, but it does not do so nearly to the extent to which we have been in 

 the habit of supposing. 



The necessary conditions for this repulsive effect to be active are, that the 

 air is acquiring heat and moisture. If the air has the same temperature as 

 our bodies, and is saturated with vapour, this force no longer exists, and gravi- 

 tation and other forces are free to act. 



Although the repulsion due to heat and evaporation are not powerful 

 enough to form a perfect protection to the lung surfaces against the contami- 

 nation of dust, yet it is very evident that their protective influence will have a 



