264 MR JOHN AITKEN ON THE 



through it, that the ascending clear space is not composed entirely of the lighter 

 gas which has come through the diaphragm. In that clear space the larger 

 proportion of the molecules are air molecules ; and while the air molecules 

 advance up to and pass through the diaphragm, the dust particles are driven 

 away from it. 1 shall presently have to refer to this. 



When speaking of the action of heat and moisture in protecting the lung 

 surfaces from contact with the suspended dust in our atmosphere, no mention 

 was made of this diffusion effect, as it can be better considered here. In our 

 lungs the small quantity of tidal air, which flows backwards and forwards, 

 carrying in the oxygen, and out the carbonic acid, never gets further than the 

 main bronchial tubes, and does not penetrate to the air-cells ; the carbonic 

 acid, set free into the residual air in these cells, is carried outwards to the tidal 

 air by diffusion, and at the same time oxygen is diffused from the tidal air 

 towards the residual air. Now, what is the effect of this diffusion on the 

 distribution of the dust % We have seen that in diffusion through a porous 

 diaphragm the dust moved towards the carbonic acid. If this was the case 

 in our lungs, then the dust would tend to penetrate towards the air-cells 

 and come into contact with their surfaces. In our lungs the exchange between 

 the carbonic acid and the oxygen does not, however, follow the law of diffusion 

 through a porous diaphragm, but those of osmose; and the rate of passage 

 of these gases through the lung surfaces does not depend upon their relative 

 densities, but on much more complicated conditions, of which solubility is in 

 this case one of the principal. The result is, that in our lungs for every volume 

 of oxygen that passes inwards, exactly or almost exactly one volume of 

 carbonic acid passes outwards. These diffusion effects balance each other, and 

 the result is that diffusion has no tendency to cause dust to penetrate towards 

 the air-cells, or to adhere to the surfaces of our lungs. 



Repulsion due to Heat. 



We shall now consider the cause of the repulsion of the dust particles by 

 hot bodies, and see if we can make out the mechanism by which the particles 

 are driven away. This is a subject of considerable difficulty, and one on which 

 I fear there will be much difference of opinion, and I shall simply state here 

 what appears to me at present to be the cause of the particles moving away 

 from a hot and towards a cold surface. The simplest explanation, and the one 

 which offered itself first, was that possibly it might be a radiation effect, and 

 that the particles are repelled in the same way as the vanes of a Crookes' 

 radiometer by the reaction of the heated gas molecules in the way explained by 

 Professors Tait and Dewae. We might suppose the side of the particles next 

 the hot surface to be warmed by radiation, and the gaseous molecules on that 



