DUST AND DISEASE. 37 



fectly certain,' the demonstration would be complete. 

 Such demonstration is furnished by experiments with a 

 beam of light. One evening, towai'ds 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, toivards the end of the expiration. 

 Permit me to repeat the experiment in your presence. 

 1 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 

 abolish this cloud by drying the breath previous to its 

 entering the beam ; or, still more simply, by warming 

 tlie glass tube. The luminous track of the beam is for 

 a time uninterrupted by the breath, because the dust 

 returning from the lungs makes good, in great part, the 

 particles displaced. After a time, however, an obscure 

 disk appears in the beam, the darkness of which in- 

 creases, until finally, towards the end of the expiration, 

 the beam is, as it were, pierced by an intensely black 

 hole, in which no particles whatever can be discerned. 

 The deeper air of Ihe lungs is thus proved to be abso- 

 lutely free from suspended matter. It is therefore in 

 the precise condition required by Professor Lister's 

 explanation. This experiment may be repeated any 

 number of times with the same result. I think it must 

 be regarded as a crowning piece of evidence both of the 

 correctness of Professor Lister's views and of the impo- 

 tence, as regards vital development, of optically pure air.* 



' Dr. Burdon Sanderson draws attention to the important 

 observation of Brauell, which shows that the contagium of a preg- 

 nant animal, suffering from splenic fever, is not found in the blood 

 of the fcetus ; the placental apparatus acting as a filter, and holding 

 back the infective particles. 



