314 FRAGMENTS OF SCIENCE. 



interesting application of the luminous beam than any 

 hitherto described. My reference to Professor Lister's 

 interpretation of the fact that air which has passed through 

 the lungs cannot produce putrefaction is fresh in your mem- 

 ories. He there assumed that the air was rendered innocu- 

 ous by the filtering action of the lungs. Can this filtering 

 process be taken out of the region of assumption and placed 

 in that of demonstration ? It can. 



Here is the concentrated beam with which we operated 

 at the commencement of this discourse. Its track through 

 the dust is luminous, and you have seen the blackness in- 

 troduced when the dust is burnt, or otherwise removed. I 

 fill my lungs with ordinary air and breathe through a glass 

 tube across the beam. The condensation of the aqueous 

 vapor of the breath is here shown by the formation of a 

 luminous white cloud of delicate texture. It is necessary 

 to abolish this cloud, and this may be done by drying the 

 breath previous to its entering the beam ; or, still more 

 simply, by warming the glass-tube. When this is done the 

 luminous track of the beam is for a time uninterrupted, 

 because the dust returning from the lungs makes good, in 

 great part, the particles displaced. After some time, how- 

 ever, an obscure disk appears in the beam, the darkness of 

 which increases, until finally, toward the end of the expira- 

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

 hole, in which no particles whatever can be discerned. The 

 deeper air of the lungs is absolutely 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 

 impotence, as regards vital development, of optically pure 

 air, 



