ON DUST AND DISEASE. 167 



electric beam sent through the air of these caves its 

 track would be invisible ; thus showing the indissoluble 

 association of the scattering of light by air and its power 

 to generate life. 



I will now turn to what seems to me a more interest- 

 ing application of the luminous beam than any hitherto 

 described. My reference to Professor Lister's interpre- 

 tation of the fact, that air which has passed through 

 the lungs cannot produce putrefaction, is fresh in your 

 memories. ' Why air,' said he, ' introduced into the 

 pleural cavity, through a wounded lung, should have 

 such wholly different effects from that entering through 

 a permanently open wound, penetrating from without, 

 was to me a complete mystery, till I heard of the germ 

 theory of putrefaction, when it at once occurred to me 

 that it was only natural that the air should be filtered 

 of germs by the air passages, one of whose offices is to 

 arrest inhaled particles of dust, and prevent them from 

 entering the air-cells.' 



Here is a surmise which bears the stamp of genius, 

 but which needs verification. If, for the words ' it is 

 only natural ' we were authorised to write ' it is per- 

 fectly certain,' the demonstration would be complete. 

 Such demonstration is furnished by experiments with a 

 beam of light. One evening, towards the close of 1869, 

 while pouring various pure gases across the dusty track 

 of a luminous beam, the thought occurred to me of 

 using my breath instead of the gases. I then noticed, 

 for the first time, the extraordinary darkness produced 

 by the expired air, towards the end of the expiration. 

 Permit me to repeat the experiment in your presence. 

 I fill my lungs with ordinary air and breathe through a 

 glass tube across the beam. The condensation of the 

 aqueous vapour of the breath is shown by the formation 

 of a luminous white cloud of delicate texture. We 



