36 THE FLOATING-MATTER OF THE AIE. 



window-frames. It tapers to a truncated cone at the 

 top. It measures in plan 3 ft. by 2 ft. 6 in., and its 

 height is 5 ft. 10 in. On February 6 it was closed, 

 every crevice that could admit dust, or cause displace- 

 ment of the air, being carefully pasted over with paper. 

 The electric beam at first revealed the dust within the 

 chamber as it did in the air of the laboratory. The 

 chamber was examined almost daily ; a perceptible 

 diminution of the floating matter being noticed as time 

 advanced. At the end of a week the chamber was 

 optically empty, exhibiting no trace of matter competent 

 to scatter the light. Such must have been the case in 

 the stagnant caves of the Paris Observatory. Were our 

 electric beam sent through the air of these caves, its 

 track would, doubtless, save from aqueous haze, be invi- 

 sible ; 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 interpie- 

 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, tiU 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 authorized to write ' it is per- 



